(un-, prefix1 affix 1.)

Not necessitated; not fixed or determined by necessity; avoidable. Also: not compelled; free.

Not obligatory.

Not essential.

Not obligatory, unforced, unnecessary.

That is a matter of choice, not compulsion; that may be done, used, chosen, etc., or not, according to a person's preference.

(un-, prefix1 affix 1.)

Of enactments, etc.: Conveying a ‘faculty’ or permission; permissive as opposed to compulsory; hence of actions, conditions, etc.: Optional.