"C" Usually 12` or 16` Box. GVW less than 26,000, usually limited to pay loads of 5,000#. Sleeper sometimes optional.
"D" Usually 22`or 24` box. GVW 33,000, Usually payloads under 13,000.Sleeper some times optional.
"E" units are semi`s
Letter "R" after any size, IE... BR,CR, DR, means...