According to the AAMVA CDL Manual, when should a bus driver wear their seat belt while driving?

AAMVA CDL Manual, Section 4.1.5, p. 4-1; 49 CFR 392.16 (a separate federal seat belt rule, not cited by the manual itself)

Bus drivers deal with constant stop-and-go traffic and intersection crashes, which the manual identifies as one of the most common causes of bus collisions. A seat belt is the driver's main protection in a crash and helps keep them seated and in control if something goes wrong on the road.