The class booking capacity is set according to the room capacity located in Admin > Configuration > Rooms. Then it can potentially be restricted by the capacity set on the class type in Admin > Scheduling/Retail > Class. Finally, it can be restricted further according to the Override Spot Count set on the class/event in the Scheduler. The "Override Spot Count" can be used to decrease the capacity for a specific class. e.g., a room has a max capacity of 25 but a yoga class will only accommodate 10 participants. You do not want to book more than 10 so you override the spot count and reduce it to 10 from 25.