A service class default is the service class that is assigned if no other classification rule matches for that subsystem type. If you want to assign any work in a subsystem type to a service class, then you must assign a default service class for that subsystem — except for STC. You are not required to assign a default service class for STC, even if you assign started tasks to different service classes.
Optionally, you can assign a default report class for the subsystem type. If you want to assign work running in a subsystem to report classes, then you do not have to assign a default service class for that subsystem.