The term is not applicable to the Air National Guard. ING members are assigned to National Guard units but do not participate in training activities. To remain members of the ING, such personnel must muster once a year with their assigned units. Those units and individuals within the Ready Reserve designated by their respective Services and approved by the Chairman of the Joint Chiefs of Staff as so essential to initial wartime requirements that they have priority over all other Reserve elements. A unit is any group or detachment of two or more individuals organized to perform a particular function, whether or not such a group is part of a larger group, and which is organized, equipped, and trained to serve, upon activation or mobilization, on active duty as an operational or augmentation unit.
Operational units train and normally serve as cohesive units.
Augmentation units train together, but lose their unit identity when activated or mobilized, normally to augment and be subsumed by an Active unit or activity. Selected Reserve units include Selected Reserve members participating in Reserve unit training activities and Active component members assigned to Selected Reserve units. A Selected Reserve unit is not limited to a minimum size. Any unit may be ordered to active duty, if needed, regardless of size. Trained individuals of the Selected Reserve who are preassigned to an Active component DoD Agency, Selective Service System or Federal Emergency Management Agency organization billet that must be filled to meet contingency, premobilization, mobilization, sustainment or demobilization manpower requirements.