A subsystem may require the services of the primary subsystem when being started. For example, it may require the primary subsystem to provide the use of subsystem data sets or an internal reader. The options service specifies whether the subsystem being added requires the primary subsystem, and is intended for use in a subsystem initialization routine.

If the START command does not specify the subsystem under which the target subsystem should start, the system uses the information specified with the REQDSUB parameter of the options request.