elective

Of, or pertaining to voting or elections; involving a choice between options.

1697, John Dryden, The Works of Virgil […] translated into English Verse, London: Jacob Tonson, dedicatory preface to the Marquess of Normanby, For his Conscience could not but whisper to the Arbitrary Monarch, that the Kings of Rome were at first Elective, and Govern’d not without a Senate:

Man thus endued with an elective voice, Must be supplied with objects of his choice.

Open to choice; freely chosen; (also, usually) unnecessary; minor.

After accounting for all of my required courses, there is hardly any room in my schedule for any elective ones.

[…] his Lordship is deceived if he think any spontaneous action after once being checked in it, differs from an action voluntary and elective, for even the setting of a mans foot, in the posture for walking, and the action of ordinary eating was once deliberated of how and when it should be done, and though afterward it became easie & habitual so as to be done without fore-thought, yet that does not hinder but that the act is voluntary and proceedeth from election.

Scheduled and nonemergent (regardless of whether necessary or unnecessary and whether minor or serious).

It was very confusing hearing my cancer surgery being classified as "elective surgery". Am I "electing" to live rather than die?

The vast majority of surgical procedures are considered elective, which means they can be scheduled in advance. […] doctors warned that thousands of patients were suffering as a result of delayed elective procedures such as hernia repair and spinal fusion. […] Nitin Yerram, director of urological oncology at Hackensack University Medical Center in New Jersey, said most cancer surgeries are considered elective, a term that's often misunderstood. "We tend to think more of tummy tucks and cosmetic surgery," Yerram said. "However, all ‘elective’ means is it's a scheduled surgery with work-up done before that. The opposite of elective is emergent or trauma situation, such as a car crash, where it's not a scheduled procedure."

Something that is an option or may be freely chosen, especially a course of study.

I still need to decide which electives to take along with my compulsory courses next semester.