conferring a faculty, privilege, permission, or the power of doing or not doing something

a facultative enactment.

left to one's option or choice; optional

The last questions in the examination were facultative.

that may or may not take place; that may or may not assume a specified character.

Biology . having the capacity to live under more than one specific set of environmental conditions, as a plant that can lead either a parasitic or a nonparasitic life or a bacterium that can live with or without air ( opposed to obligate ).

of or pertaining to the faculties.

adjective

discretionary , elective