As a basic physical/operational characteristic a CDL holder must understand, what passenger-capacity threshold makes a vehicle, including a school bus, subject to CDL requirements regardless of its weight?

49 CFR 383.91 (CDL vehicle groups, Group C: designed to transport 16 or more passengers, including the driver, or used to transport hazardous materials)

Understanding why a particular school bus requires a CDL, not just that it does, is part of knowing the vehicle's operational characteristics. The 16-passenger threshold exists because safely moving that many people, especially children, demands a higher standard of driver knowledge and skill than the vehicle's weight alone would require.