The third category is related business codes, such as personal identification code (PIN), unlock code (PUK), billing rate, etc. The fourth category is the telephone directory. Almost all user information is stored in the SIM card, so the SIM card is also called "user information identification card". SIM card can be divided into big card and small card. The small card is called "plug-in SIM card", which is the chip on the big card.

SIM card is called "user identification card". It is actually a smart card containing large-scale integrated circuits, which is used to register important data and information of users. The most important function of SIM card is authentication and encryption. When a user moves to a new area to make or receive a call, the switch must authenticate the user to determine whether it is a legal user.