Holdfold it varies from one area to another, in my area there is what is called a "24 hour", "Serentity", or "Surrender" chip which is what a newcomer to AA can pick up, basically it is kind of a physical symbol of wanting to start a new way of life.
Then there are the monthly chips which is basically to be picked up when one has a certain number of months of continuous sobriety, the main reason these are offered is more to be a symbol of hope to people with less time that they can see others stay sober. We give out monthly chips for 1 thru 11 months, then we give out a one year chip as well as an 18 month chip, all other chips are in yearly increments.
The primary purpose in any chip given out is to give hope to those with less time that it is possible to stay sober for many years.
Picking up chips is not required, I know one guy who did not pick up a single chip until he had 5 years sober, he shared that the only reason he started to pick up chips was to give hope to others.