Each service branch offers full- and part-time service commitments to their members, dividing their forces into three subsets: active duty, which is full-time, and reserve and national guardsmen, which are part-time.
The active duty component is made up of service members whose full-time job is serving in the military, while reserve and National Guard components are made up of individuals who generally perform a minimum number of duty days per year and supplement the active duty military when necessary.
For civilian employers, the important differentiator between the full-time and part-time components is that individuals who serve in a part-time capacity (be they reservists or national guardsmen) typically hold full-time civilian jobs in addition to their monthly and yearly military obligations. This makes them potential candidates for jobs while they continue to serve in the military and gives them skills and experience gained in tandem from both their civilian job and their military job.
On the other hand, individuals who serve full-time in the active duty component will become potential job candidates upon transition out of the active component. This can mean that they are retiring or leaving the military after their term of service or are transitioning to a part-time service component (e.g., moving from active duty to being a reservist).