There are five types of users in sap

Dialog users (A)

A normal dialog user is used for all logon types by exactly one person. This is used to logon using SAP GUI. During a dialog logon, the system checks for expired/initial passwords. The user can change his or her own password. Multiple dialog logons are checked and, if appropriate, logged. These users are used for carrying out normal transactions. This is an interactive type of logon. The initial multiple logons are 6. They are set according to companies policy.

System Users (B)

These are non interactive users. They are used for background processing and internal communication in the system (such as RFC users for ALE, Workflow, TMS, and CUA). Their passwords cannot be changed by the end users. Only the user administrator can change their passwords. Multiple logon is permitted in these type of users. Dialog logon is not possible for these type of users.

Communication Users (C)

Used for dialog-free communication between systems. It is not possible to use this type of user for a dialog logon. Their passwords are valid for certain period of time so they expire. The users have option to change their own passwords.

Service User (S)

Dialog user available to a larger, anonymous group of users. The system does not check for expired/initial passwords during logon. Only the user administrator can change the passwords. Generally, highly restricted authorizations are given to this type of users.

Reference User (L)

A reference user is, like the service user, a general non-person-related user. Dialog logon is not possible with this kind of user. A reference user is used only to assign additional authorizations. To assign a reference user to a dialog user, specify it when maintaining the dialog user on the Roles tab page