| optional(adjective, open, chance, choice, freedom) |
| discretional(adjective, open, free, will, voluntary) |
| elective(adjective, open, free, fit, choice) |
| arbitrary(adjective, open, free, chance, opinion) |
| facultative(adjective, open, free, choice, voluntary) |
| voluntary(adjective, open, free, will, elective) |
| eligible(adjective, fit, worthy) |
| nonmandatory(adjective, open) |
| unrestricted(adjective, open, free, freedom, voluntary) |
| open(adjective, open, free, extra, responsive) |
| absolute(adjective, freedom) |
| flexible(adjective, responsive, variable, adaptable) |
| free(adjective, open, will, voluntary, elective) |
| nonobligatory(adjective, open, free, voluntary, elective) |
| unforced(adjective, open, free, voluntary, elective) |
| spontaneous |
| uncoerced |
| volitional(open, free, will, voluntary, elective) |
| volunteer |
| willful |