Supplemental Payroll payments consist of payments not paid through the normal Form 500 process. A listing of the authorized types is included in ...

A supplemental payment is an employee salary payment submitted when the employee did receive payment from a regular biweekly or regular monthly payroll but is owed additional monies beside what the employee has already received.

...supplemental payments of compensation made to university employees for services that they provide to the university, which are outside the normal scope of their employee appointment...

http://www.dof.ca.gov/html/calstars/calsdocs/manual/volume-2...
The Supplemental Payroll Account are charged with types of payments other
than regular time, overtime, shift differential, and shift differential overtime. Lump sum payments and retroactive salary adjustments are two examples of the types of payments that are charged to the Supplemental Payroll Account. The charges in this account may be distributed manually or automatically through the cost allocation process.