SAP User Types

User types
Dialog 'A'
A normal dialog user is used by exactly one person for all logon types.
Dialog logons are checked for obsolete/initial passwords which must be changed.
Multiple dialog logons are checked and logged.

System 'B'
Use the user type System for dialog-free communication within one system. (for RFC or CPIC service users) or for background processing in one system.
Dialog logon is not possible.
A user of this type is excluded from the standard settings for password validity period. The password can only be changed by user administrators or in transaction Su01 (Goto -> Change Password)

Communication 'C'
Use the user type Communikation for dialog-free communication between systems (for RFC or CPIC service users of different applications, for example, ALE, Workflow, TMS ZBV).
Dialog logon is not possible.

Service 'S'
A user of type Service is a dialog user available to a large anonymous set of users. It usually has closely-restricted authorizations.
Service users are e.g. used for anonymous system access via an ITS service. You can change a session which began as an anonymous session with a service user into a personal session under a dialog user with an individual authentification.
There is no check for obsolete/initial passwords at logon. Only the user administrator can change the password.
Multiple logon is allowed.

Reference 'L'

A Reference user is a general impersonal user like the Service user. You cannot logon with a Reference user. The Reference user is to give Internet users identical authorizations.
You can specify a Reference user for additional dialog user authorizations, in the Roles tab. The application generally controls the assignment of Reference users. The name of the Reference user can be assigned in variables which should begin with "$". The assignment variable-Reference user is made in the transaction SU_REFUSERVARIABLE.
This assignment applies to all systems in a CUM landscape. If the assigned Reference user does not exist in a CUM subsidiary system, the assignment is ignored.

System
Use the user type System for dialog-free communication between systems (for RFC or CPIC service users) or for background processing in a system. Dialog logon is not possible.
A user of this type is excluded from the general password validity period settings. The password can only be changed by the user administrator in the transaction SU01 under Goto -> Change Password.

We recommend assigning the appropriate user type when creating users. For example, if the user does not need dialog access to the SAP System, then define it as a system user. If the user is an anonymous, public user that many different individuals can use, then define it as a service user and keep its authorizations to a minimum.