I think you mean tcode SU01. Here is the help info for System user:
Use the system user type for internal system processes (-> background processing) or system-related processes (-> ALE, workflow, TMS, CUA).
Dialog logon (using SAP GUI) is not possible.
A user of this type is excluded from the general settings for password validity. Only user administrators can change the password using transaction SU01 (Goto -> Change Password).
Multiple logons are permissible.
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