voluntary

(military) a person who freely enlists for service

composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a religious service or musical performance

of your own free will or design; done by choice; not forced or compelled; "man is a voluntary agent"; "participation was voluntary"; "voluntary manslaughter"; "voluntary generosity in times of disaster"; "voluntary social workers"; "a voluntary confession"

controlled by individual volition; "voluntary motions"; "voluntary muscles"