8 C.F.R. § 315.1
As used in this part:
Exemption from military service means either:
(1) A permanent exemption from induction into the Armed Forces or the National Security Training Corps of the United States for military training or military service; or
(2) The release or discharge from military training or military service in the Armed Forces or in the National Security Training Corps of the United States.
Induction means compulsory entrance into military service of the United States whether by conscription or, after being notified of a pending conscription, by enlistment.
Treaty national means an alien who is a national of a country with which the United States has a treaty relating to the reciprocal exemption of aliens from military training or military service.
8 C.F.R. §315.1