rdisp/start_service_user is a SAP Parameter attribute which is used to control ABAP user to run the runlevel system startup information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
The SAP Start Service requires a valid ABAP logon for certain actions
(such as changing a parameter). You can use this parameter to configure
the ABAP user that is used for the logon.
Caution:
If the parameter is not explicitly set, the system uses the value of the
parameter rdisp/abap_user_for_sidadm.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.