| optional(adjective, possible, voluntary, elective) |
| alternative(open, free, voluntary, elective) |
| arbitrary(open, free, voluntary, elective) |
| discretional(open, free, voluntary, elective) |
| discretionary(open, free, voluntary, elective) |
| elective(open, free, voluntary) |
| extra(open, free, voluntary, elective) |
| facultative(open, free, voluntary, elective) |
| free(open, voluntary, elective) |
| no strings attached(expression, idiom, open, free, voluntary) |
| noncompulsory(open, free, voluntary, elective) |
| nonobligatory(open, free, voluntary, elective) |
| not required(expression, open, free, voluntary, elective) |
| open(free, voluntary, elective) |
| possible(voluntary, elective) |
| uncompelled(voluntary, elective) |
| unforced(open, free, voluntary, elective) |
| unrestricted(open, free, voluntary, elective) |
| volitional(open, free, voluntary, elective) |
| voluntary(open, free, elective) |