Since 1978, all amateur radio station call signs have been initially assigned in strict sequential order according to a list of four major groups. These groups are simply called: Group A, Group B, Group C and Group D. The license class that the applicant qualifies for determines the Group.
Extra Class amateurs qualify for the shorter (and theoretically more desirable) Group A (1x2, 2x1 and "A" prefixed 2x2) call signs; Advanced Class amateurs qualify for Group B (2x2) call signs, Technician, Tech Plus and General Class amateurs qualify for Group C (1x3) call signs and Novices for the longer (2x3 format) Group D calls. Amateur station call signs consist of three sections:
a one or two letter prefix, a single area numeral (zero through 9) and a one, two or three letter suffix.
The FCC has an online information bulletin entitled The Sequential Call Sign System.