The Atthasalini (I, Part  IV, Chapter 1, 110) states about sanna:

...It has the characteristic of noting (In Pali: sanjanati, cognizing well) and the function of recognizing what has been previously noted. There is no such thing as perception in the four planes of existence without the characteristic of noting. All perceptions have the characteristic of noting. Of them, that perceiving which knows by specialized knowledge has the function of recognizing what has been noted previously. We may see this procedure when the carpenter recognizes a piece of wood which he has marked by specialized knowledge...

   The Atthasalini then gives a second definition: 

   Perception has the characteristic of perceiving by on act of general inclusion, and the function of making marks as a condition for repeated perception (for recognizing or remembering) (I am using the translation of the ven. Nyanaponika, Abhidhamma Studies, page  69, BPS, Kandy, 1976), as when woodcutters 'perceive' logs and so forth. Its manifestation is the action of interpreting by means of the sign as apprehended, as in the case of blind persons who 'see 'an elephant (3 Here I use the English translation of the Visuddhimagga, XIV, 130, instead of the English text of the Atthasalini, the commentary refers to a story in the "Udana" (Verses of Uplift, Minor Anthologies, 68-69) about blind people who touch different parts of an elephant. Each of them interprets in his own way what an elephant is Iike: the Person who touches the head believes that the elephant is Iike a pot, since he remembers whet a pot is Iike; the person who touches the  manifestation, like lightning, owing to its inability to penetrate the object. Its proximate cause is whatever object has appeared, like the perception which arises in young deer mistaking scarecrows for men.) .Or, it has briefness as tusks believes that it is Iike a ploughshare, and so on. Thus, there is recognizing of a sign or label which was made before.

The Visuddhimagga (XIV, 130) gives a similar definition. We can use the words perceiving, noting, recognizing and 'marking' in order to designate the reality which is sanna, but words are inadequate to describe realities. We should study the characteristic and function of sanna.

     Sanna is not the same as citta which is the 'leader' in cognizing an object. As we have seen, sanna recognizes the object and it 'marks' it so that it can be recognized again. This is explained by way of a simile: carpenters put tags or signs on logs so that they can recognize them at once by means of these marks. This simile can help us to understand the complex process of recognizing or remembering. What we in conventional language call "remembering" consists of many different moments of citta and each of these moments of citta is accompanied by sanna  which connects past experiences with the present one and conditions again recognition in the future. This connecting function is represented by the words 'recognition' and 'marking' (1 See Abhidhamma Studies, by the Ven. Nyanaponika, 1976, page 70, where it is explained that the making of marks and remembering is included in every act of perception.)  when the present experience has fallen away it has become past and what was future becomes the present, and all the time there is sanna which performs its  function so that an object can be recognized. If we remember that sanna accompanies every citta, we will better understand that the characteristic of sanna is not exactly the same as what we mean by the conventional terms of 'recognition', 'perception' or 'marking' . Each citta which arises falls away immediately and is succeeded by the next citta, and since each citta is accompanied by sanna which recognizes and 'marks 'the object, one can recognize or remember what was perceived or learnt before. 

       The Atthasalini mentions as a manifestation of sanna:
       'briefness, like lightning, owing to its inability to penetrate the object'.

   Sanna  merely recognizes and 'marks' the object. Sanna is different from citta which is the 'chief in knowing an object and different from panna which can know the true nature of realities. The proximate cause of sanna is an object, in whatever way that appears. The object can be a paramattha dhamma, i-e nama or rupa, or a concept (pannatti). Whatever object citta cognizes, sanna recognizes and marks it. Sanna performs its function through each of the six doors. There is sanna at this moment. When there is seeing there is sanna and it recognizes and marks visible object. When there is hearing there is sanna which recognizes and marks sound. There is sanna when there is smelling, tasting, touching or when there is the experience of objects through the mind-door. cittas experience objects through the six doors and the sanna which accompanies citta experiences the object through the same doorway and performs its function accordingly. When we recognize someone's voice, this is actually the result of different processes of cittas which experience objects through the sense-door and through the mind-door. At are moment there is sanna which performs its function. There are moments of hearing of what appears through the ears, of sound, and when we think of someone's voice there are cittas which experience concepts. The hearing conditions the thinking, we could not think of a voice if there were not hearing. It is the same when we think we 'see' a person. There is thinking of a concept, but is thinking is conditioned by the seeing of visible object, The recognition of a person is the result of many different processes of citta and each moment of citta is accompanied by sanna. There is seeing which experiences visible object and after the eye-door process has been completed visible object is experienced through the mind-door There are other mind-door processes of cittas which experience concepts. Sanna accompanies every citta and also when citta experiences a concept sanna marks and remembers that object. When we are engaged in the activities of our daily life, do we notice that there is recognition or remembrance?  We remember how to use different objects, how to eat which fork, knife and spoon, how to turn on the water tap, how to write or how to find our way when we walk in our house or on the street. We take it for granted that we remember aII these things. We should know that it sanna which remembers

   The Visuddhimagga (XIV, 3-5) explains the difference between, citta and panna by way of a simile. Sanna is like the mere perception of a coin by a child who does not know its value.  Citta is like the villager who knows its value. Panna is like the money-changer who penetrates its true characteristics.

   When we are reading it is due to sanna that we recognize the letters and know their meaning. However, we should not forget that when we are reading there are also moments of seeing and at such moments sanna performs its function as well. it seems that we see and recognize what we see all at the same time, but this is not so. When we recognize letters and words and remember their meaning, this is not due to one moment of sanna but to many moments of sanna accompanying the cittas which succeed one another in the different processes. The study of sanna can remind us that cittas arise and fall away extremely rapidly. Countless moments of sanna succeed one another and perform their function so that we can remember. successive events such as sentences we hear when someone is speaking. There are moments of hearing and the sanna which accompanies hearing-consciousness merely perceives the sound, it does not know the meaning of what is said. When we understand the meaning of what has been said there are cittas which experience concepts and the sanna which accompanies those cittas remembers and 'marks' a concept. Because of many moments of sanna we can follow the trend of thought of a speaker or we ourselves can reason about something, connect parts of an argument and draw conclusions. All this is not due to 'our memory' but to sanna which is not self but only a kind of nama. What we take for 'our memory' or 'our recognition' is not one moment which stays, but many different moments of sanna which arise and fall away. Because of sanna past experiences and also concepts and names are remembered, people and things are recognized. Also when we do not remember something or we mistake something for something else, there is sanna which accompanies the cittas at such moments. If we have forgotten something, we did not think of the object we wanted to think of but at that moment we were thinking of another object and this was remembered and marked by sanna. For example, if we go to the market and forget to buy lettuce because we suddenly notice tomatoes and our attention turns to the tomatoes, we say that we have forgotten to buy lettuce. In reality there are moments of sanna all the time since it accompanies each citta, and sanna performs its function all the time. it depends on conditions what object is remembered at a particular moment, it does not always turn out the way 'we' want it. Also when we in vain try to remember a name, there is still sanna, but it remembers and 'marks' an object which is different from the concept we think we should remember. We may have aversion because of our forgetfulness and also then there is citta accompanied by sanna which performs its function. Sanna accompanies cittas which arise in a process and it also accompanies cittas which do not arise in a process, namely the patisandhi-citta (rebirth-consciousness), the bhavanga-citta (life-continuum) and the cuti-citta (dying-consciousness). When we are sound asleep and not dreaming them are bhavanga-cittas and also in between the different processes of cittas there are bhavanga-cittas. The object of the patisandhi-citta, the bhavanga-citta and the cuti-citta is the same as the object experienced by the javana-cittas which arose shortly before the cuti-citta of the previous life (Abhidhamma in Daily Life, Chapter 15.) 'We', or rather the cittas which are thinking at this moment, do not know what that object is. However every time the bhavanga-citta arises in between the processes of cittas it experiences that object and the sanna which accompanies the bhavanga-citta remembers that object. Sanna never arises alone, it has to accompany citta and other cetasikas and it is conditioned by them. Sanna is sankhara dhamma, conditioned dhamma. Sanna arises with the citta and then falls away with the citta, sanna is a khandha, it is one among the five khandhas. We cling to sanna, we take it for self. Sanna arises with all cittas of the four jatis. Sanna is of the same jati as the citta it accompanies and thus sanna can be akusala kusala, vipaka or kiriya. Sanna can be classified according to the six kinds of objects which are experienced through the six doors and this reminds us that sanna is different all the time. We read in the Gradual Sayings (Book of the sixes, Chapter VI, 9, A Penetrative Discourse):

   "Monks, perceptions are six: perceptions of visible objects, sounds, smells, tastes, touches and ideas."

   The perception of visible object is not the perception of sound and it is not the perception of a concept. When we for example talk to someone else there is sanna which perceives sound, there is sanna which perceives visible object, there is sanna which perceives tangible object, there it sanna which perceives a concept. All these sannas are completely different from one another and they arise at different moments. Objects appear one at a time through the different doorways and different sanna mark and remember these objects. When we understand this it will help us to see that our life actually is one moment of citta which experiences one object through one of the six doors. The ultimate truth is different from conventional truth, namely, the world of people and things which seem to last. 

     Sanna which arises with akusala citta is also akusala. Sanna may arise together with wrong view. When one takes for permanent what is impermanent the citta with wrong view is also accompanied by sanna which remembers the object in a distorted way. It is the same when one takes for self what is not self. We read in the Gradual Sayings, Book of the Fours, chapter V, 9, Perversions about four perversions (vipallasas) of sanna, citta and ditthi:

Monks, there are these four perversions of perception (sanna), four perversions of thought (citta), four perversions of view (ditthi). What four?

     To hold that in the impermanent there is permanence, is a perversion of perception, thought and view. To hold that in dukkha there is non-dukkha, is a perversion of perception, thought and view. To hold that in the not-self there is self, is a perversion of perception, thought and view. To hold that in the foul there is the fair, is a perversion of perception, thought and view. These are the four perversions of perception, thought and view...

……Visual consciousness, your reverences, arises because of eye and visual object; the meeting of the three is sensory impingement (phassa); feelings are because of sensory impingement; what one feels one perceives; what one perceives one reasons about; what one reasons about obsesses one; what obsesses one is the origin of the number of perceptions and obsessions which assail a man in regard to visual object cognisable by the eye, past, future, present...

     The fourth stage of arupa-jhana is the 'Sphere of neither perception nor non-perception' (n'eva-sanna-n'asannayatana) (Abhidhamma in Daily Life chapter  22.). The sanna which accompanies the arupavacara citta of the fourth stage of jhana is extremely subtle. we read in the Visuddhimagga (X,50):

   the perception here is neither perception, since it is incapable of
performing the decisive function of perception, nor yet non-perception,
since it is present in a subtle state as a residual formation. thus it is
neither perception nor non-perception.. ( Abhidhamma in Daily Chapter 22.)

   Monks. these ten ideas, if made to grow and made much of are of great fruit, of great profit for plunging into the deathless. for ending up in the deathless. What ten ideas? The idea of the foul, of death, of repulsiveness in food, of distaste for all the world, the idea of impermanence, of dukkha in impermanence, of not-self in dukkha, the idea of abandoning, of fading, of ending. These ten ideas, monks, if made to grow... are of great profit for plunging into the deathless, for ending up in the deathless. 

i    Sanna accompanies each citta, but it falls away completely with the citta. How can we still remember things which happened in the past?
ii    When we see a house, through which doorway does sanna perform its function? 
iii   When we mistake something for something else, how can there still be sanna at such a moment?
Iv   When we recognize a house, can there be perversion of sanna?
v    Can the sotapanna think of concepts and recognize people and things? 
Vi  Give examples of akusala sanna.
Vii How can one develop 'perception of impermanence' (anicca sanna)?