Officers may be commissioned upon graduation from the United States Air Force Academy, upon graduation from another college or university through the Air Force Reserve Officer Training Corps (AFROTC) program, or through the Air Force Officer Training School (OTS). OTS, previously located at Lackland AFB, Texas until 1993 and located at Maxwell Air Force Base in Montgomery, Alabama since 1993, in turn encompasses two separate commissioning programs: Basic Officer Training (BOT), which is for line-officer candidates of the active-duty Air Force and the U.S. Air Force Reserve; and the Academy of Military Science (AMS), which is for line-officer candidates of the Air National Guard. (The term "line officer" derives from the concept of the line of battle and refers to an officer whose role falls somewhere within the "Line of the Air", meaning combat or combat-support operations within the scope of legitimate combatants as defined by the Geneva Conventions.)
What is a line officer in the USAF? 
concept of the line of battle and refers to an officer whose role falls somewhere within the "Line of the Air"