From the DoD Dictionary of Military and Related Terms

(DOD) A tour of active duty which is used for training members of the Reserve Components to provide trained units and qualified persons to fill the needs of the Armed Forces in time of war or national emergency and such other times as the national security requires. The member is under orders that provide for return to non-active status when the period of active duty for training is completed. This includes annual training, special tours of active duty for training, school tours, and the initial duty for training performed by nonprior service enlistees. Also called ADT.

From this source:

Sadly, few of today's partisan pundits know anything about the environment of service in the Reserves in the 1970s. The image of a reservist at that time is of one who joined, went off for six months' basic training, then came back and drilled weekly or monthly at home, with two weeks of "summer camp." With the knowledge that Mr. Johnson and Mr. McNamara were not going to call out the Reserves, it did become a place of refuge for many wanting to avoid Vietnam.

There was one big exception to this abusive use of the Guard to avoid the draft, and that was for those who wanted to fly, as pilots or crew members. Because of the training required, signing up for this duty meant up to 2½ years of active duty for training alone, plus a high probability of mobilization. A fighter-pilot candidate selected by the Guard (such as Lt. Bush and me) would be spending the next two years on active duty going through basic training (six weeks), flight training (one year), survival training (two weeks) and combat crew training for his aircraft (six to nine months), followed by local checkout (up to three more months) before he was even deemed combat-ready. Because the draft was just two years, you sure weren't getting out of duty being an Air Guard pilot. If the unit to which you were going back was an F-100, you were mobilized for Vietnam. Avoiding service? Yeah, tell that to those guys.

The Bush critics do not comprehend the dangers of fighter aviation at any time or place, in Vietnam or at home, when they say other such pilots were risking their lives or even dying while Lt. Bush was in Texas. Our Texas ANG unit lost several planes right there in Houston during Lt. Bush's tenure, with fatalities. Just strapping on one of those obsolescing F-102s was risking one's life.

Like all members of the National Guard and Reserves, Bush had to go on ADT status while in training. In his case, that included flight school. He was still a member of the National Guard but was on ADT status for something like 2 years. Legally, there are some distinctions between Active Duty and Active Duty for Training status. While his uniform would've said "U. S. Air Force", his ID card would've listed him as a member of the national guard.