Could be both a and b, the locals out of Bay Plaza after 9:30pm tend to pick up people while the SBS is still running (they may not be sure the SBS is still running at that time either).
If the SBS is usually crowded, that means more service will eventually be added, even if the local is empty. MTA could run the SBS another hour in the late evening into early late night and have enough ridership to justify it (especially if the local is cut back to Pelham Bay another hour in the late evening).
If SBS could run about two hours later, the local wouldn't really have to run to/from Bay Plaza at all, just run to Pelham Bay, until the last few SBS trips, then late night service to Edson Avenue.