Class Category
1-A Available for unrestricted military service.
1-A-O Conscientious objector available for noncombatant military service only.
1-C Member of the Armed Forces of the United States, the National Oceanic and Atmospheric Administration or the Public Health Service.
1-D-D Deferment for certain members of a reserve component or student taking military training.
1-D-E Exemption of certain members of a reserve component or student taking military training.
1-H Active Registrant (*All registrants currently in the database have this classification)
1-O Conscientious objector to all military service. A registrant must establish to the satisfaction of the board that his request for exemption from combatant and noncombatant military training and service in the Armed Forces is based upon moral, ethical or religious beliefs which play a significant role in his life and that his objection to participation in war is not confined to a particular war.
1-O-S Conscientious objector to all military service.
1-W Conscientious objector ordered to perform alternative service.
2-D Registrant deferred because of study preparing for the ministry.
3-A Registrant deferred because of hardship to dependents.
3-A-S Registrant deferred because of hardship to dependents (separated).
4-A Registrant who has completed military service.
4-A-A Registrant who has performed military service for a foreign nation.
4-B Official deferred by law.
4-C Alien or dual national.
4-D Minister of religion.
4-F Registrant not acceptable for military service. To be eligible for Class 4-F, a registrant must have been found not qualified for service in the Armed Forces by a Military Entrance Processing Station (MEPS) under the established physical, mental, or moral standards. The standards of physical fitness that would be used in a future draft would come from AR 40-501. Openly gay men are not allowed to serve in the American military.
4-G Registrant exempted from service because of the death of his parent or sibling while serving in the Armed Forces or whose parent or sibling is in a captured or missing in action status.
4-T Treaty alien.
4-W Registrant who has completed alternative service in lieu of induction.
Back in the day there were also:
I-Y - available only in national emergency - this was the typical what you got if you were medically disqualified. It was subject to review on a 6-month or annual basis. After two or three years, if you weren't reclassified I-A, they'd switch it to a 4-F.
II-S - college student
I know it's the current list, I posted a link to it earlier.