Typical thread class usage:
A class 1A external thread and a class 1B internal thread gives a loose fit.
A class 2A external thread and a class 2B internal thread gives a normal fit.
A class 3A external thread and a class 3B internal thread gives a close fit.
Classes 1A, 2A, and 3A gages are thread ring gages to check external threads.
Classes 1B, 2B, and 3B gages are thread plug gages to check internal threads.
