They are the principal United States Air Force liaison element to the United States Army (USA) provided in accordance with the Memorandum of Agreement between the U.S. Air Force and the U.S. Army for the Exchange of Liaisons. In this context, the TACP is an Air Force liaison element aligned with Army combat maneuver echelons from corps to battalion. The TACP provides its aligned Army unit with expertise in planning and executing airpower in support of the land component commander's scheme of maneuver.
The USAF TACP, when operationally employed as an element of the Theater Air Control System (TACS), is subordinate to the Air Support Operations Center (ASOC), which in turn is subordinate to the Air Operations Center (AOC). The AOC is the senior TACS agency responsible for the centralized control and decentralized execution of airpower in support of the Joint Force Commander. To accomplish its mission, the TACP element has several different AFSCs. These AFSCs will vary depending on which Army echelon the TACP supports and they are identified in the USAF/USA Memorandum. The USAF/USA Memorandum requires the USAF to provide Air Liaison Officers, Battalion Air Liaison Officers, enlisted technicians (1C4X1s) skilled in planning, requesting, and managing airpower resources, and 1C4X1 Joint Terminal Attack Controllers (JTACs). JTACs are specially trained and certified airmen who provide terminal control of airpower, usually in the form of Close Air Support missions. In accordance with the USAF/USA Memorandum, the TACP also provides USAF Intelligence, Space, Electronic Warfare, Weather, and other liaisons to the Army. These liaisons serve as USAF subject-matter experts within their areas of expertise and assist in planning and integrating these functions with their aligned Army unit. All of these USAF liaison personnel are assigned to a USAF Air Support Operations Squadron. Operationally, liaisons serve within a TACP aligned with an Army Brigade Combat Team (BCT, division, or corps. 1C4X1s and Air Liaison Officers serve in TACPs at Army echelons from battalion through corps.
The US Special Operations Command was established in 1987 at MacDill AFB, FL. One MSgt. 1C4X1 (then AFSC 275X0) was assigned to the command as a liaison, but the position was disestablished in approximately 1991. In 1997, two MSgt. JTAC-qualified 1C4X1s assigned to the 17th ASOS at Ft. Benning, GA were selected for assignment to two of the Air Force Special Operations Command's Special Tactics Squadrons. Since then, the 17th ASOS, which provides JTACs and ALOs to the 75th Ranger Regiment and its three line battalions, has transferred from Air Combat Command to Air Force Special Operations Command and in July 2013 was renamed 17th Special Tactics Squadron. JTAC-qualified 1C4X1s at the "5" or "Craftsman" skill level may apply for a Special Tactics assignment within Air Force Special Operations Command (AFSOC). After a rigorous selection process and training program, AFSOC 1C4X1 JTACs provide terminal attack control and fire support expertise for the three Ranger Battalions, the 75th Ranger Regiment's Reconnaissance Company, all seven Army Special Forces Groups, multiple Naval Special Warfare Groups (SEAL Teams), and all four active duty CONUS Special Tactics Squadrons.
The above information provided in part by Wikipedia, The Institute of Heraldry, Global Security, and the official websites of the corresponding units and formations.