How endorsements work
The U.S. Commercial Driver's License system is built on top of a "base" Class A, B, or C CDL. The base license alone permits operation of certain vehicle categories, but to legally haul hazardous materials, drive a tank truck, pull double or triple trailers, transport passengers, or operate a school bus, the driver must add a corresponding endorsement to that base license.
Endorsements are designated by single letters: H for Hazardous Materials, N for Tank Vehicles, T for Doubles/Triples, P for Passenger, S for School Bus, and combined codes such as X for Hazmat-and-Tank. The Air Brakes restriction (the absence of an "L" restriction) and the Combination Vehicles base material are not technically endorsements, but they are tested as separate written exams and are usually grouped with endorsement preparation in the popular study sites.
Practical advice: take the General Knowledge exam first, then study Air Brakes and Combination at the same time if you intend to drive a tractor-trailer. The big-money endorsements (H, N, X, P, S) are best earned one at a time, with focused study, because each pulls from a meaningfully different section of the federal CDL Manual and each carries different downstream consequences (TSA background check, additional skills test, etc.).
The pages linked above include the full federal-pool practice questions for each exam, organized by topic and presented with detailed answer explanations. None of the practice questions on this site replace the official state CDL Manual — but they do reflect the same source material your state used to write its test.