rdisp/system_startup_user is a SAP Parameter attribute which is used to control Specifies the user for system startup processing 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 .
All tenants are switched to the required target level using the tenant
runlevel framework during system startup. These activities include a
kernel and an ABAP part. The actual tenant activities take place in the
ABAP part. This part must therefore take place with an ABAP user that is
authorized to execute tenant runlevel operations.
This user can be set up using parameter rdisp/system_startup_user. Users
configured by parameter
See SAP Parameter Documentation for full SAP documentation for this profile parameter.