SYSVAR returns information about MVS, TSO/E and the current session, such as levels of software available, your logon procedure, and your user ID.
The information returned depends on the arg_name value specified on the function call.
SYSPREF is an arg_name. SYSVAR("SYSPREF") returns the prefix as defined in the user profile. The prefix is usually the user's user-id. However, TSO/E PROFILE command can be used to change the prefix.