nonmandatory (adj.)

• arbitrary
• autonomous
• discretional
• discretionary
• elective
• free
• gratuitous
• independent
• optional
• self-acting
• spontaneous
• unasked
• uncalled-for
• uncoerced
• unforced
• uninfluenced
• uninvited
• unprompted
• unrequested
• unsolicited
• unsought
• voluntary
• volunteer
• willful