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.

Specifically, its functions mainly reflect the following aspects. The data stored by SIM card can be divided into four categories: the first category is fixed data. This type of data is written by the SIM card center before the mobile phone is sold, including the International Mobile Subscriber Identification Number (IMSI), authentication key (KI), authentication and encryption algorithm, etc.