P = Passenger Vehicles designed to transport more than 15 persons (including the driver). Requires completion of the written test, plus a road test in a passenger bus of the class for which permission to operate is being sought (if you have, for example, a Class A CDL, but you road test in a bus which requires a Class B CDL, then you're restricted to passenger vehicles requiring a Class B or Class C CDL).
S = Passenger endorsement plus completion of a background check for the transport of school children in a bus. Requires completion of the passenger endorsement test, a road test in a passenger bus, and completion of the aforementioned background check.
T = Double and triple trailers. Requires only a written test, which is basically a rehash of the combination vehicles written test.
N = Tanker. Requires only a written test.
H = Hazardous materials endorsement. Requires completion of the written test plus a TSA background check.
X = In some states, when a driver has both hazmat (H) and tanker (N) endorsements, the two endorsements combined will appear as an 'X' endorsement.
