There are some specialized types of vehicles such as buses or tank trucks and in order to drive them or carry hazardous materials, you are required to apply for the proper endorsement on your CDL.
Endorsements include Passenger (P), allowing you to carry passengers;
Tank (T), allowing you to drive trucks containing liquid cargo;
H (Hazardous Materials), allowing you to drive trucks containing such hazardous materials as flammable liquids, explosives, or radioactive substances. To obtain an endorsement, you will need to pass a specialized knowledge exam and possibly a specialized driving skills test.
To obtain an H, P, or School Bus (S) endorsement, you must successfully complete training from FMCSA-approved training providers. To obtain an S endorsement, you must also undergo a thorough background check.