Hello Kyle,
Good question. There is a small difference between the two.
The 999s were borrowed from the rail side to indicate a cancelled or suspended vehicle.
The 998s are a little different in that it's a trip without a vehicle ID. According to the schedule, it should be running, but there's no real-time data for it. All 998s have a VehicleID of 0. The reason might vary: the GPS unit could be malfunctioning, the driver might not have been able to log in, etc. but all we know is it's not cancelled (yet). It may or may not be out in service. It's Schrödinger's bus, if you will. In the mobile app, a 998 is listed as 'untracked'.
-Greg