elective
ex gratia
facultative
nonmandatory
nonobligatory
alternative
arbitrary
autonomous
discretional
discretionary
disjunctive
free
free will
gratuitous
independent
offered
proffered
self-acting
self-active
self-determined
self-determining
spontaneous
unasked
unbesought
unbidden
uncalled-for
uncoerced
uncompelled
unforced
uninfluenced
uninvited
unpressured
unprompted
unrequested
unrequired
unsolicited
unsought
volitional
voluntary
volunteer
willful
option
optionality
additional
analog
audio
built-in
correspond
diagnostic
dual
functional
graphical
high-speed
numeric
optical
plug-in
programmable
stand-alone
wireless
compulsory
mandatory
obligatory