Extra Duty........is *generally* meant as a punitive term.
You go to Capt's Mast and are stripped of one rank, perhaps fined, and most assuredly, assigned "extra duty". (if not outright receiving brig time)
Fines of Extra Duty are usually 15 or 30 days......you are restricted to the ship, and after the "normal" work day, you have to muster with the Master of Arms, and they will ensure you perform some menial task.......cleaning P-ways, polishing bright work, that sort of thing.
People assigned to the BRIG.....will also be let out for this "extra duty". ...and then returned to the brig.
Additional duties .......can be chores or assignments above and Beyond your normal job.
You could be tasked with helping with a working party. It's NOT a punishment.......it's simply a task that requires more hands....and lucky you, if you get tapped.
You could be asked to stand someone else's watch........for whatever reasons. ....maybe they are sick or have been injured and the shift needs to be covered at the last second.
You can volunteer for additional duty........like working for the COM-REL committe, like I did.
Not required....they can't make you........it's strictly voluntarily for those who wish to work on community projects in foreign ports, instead of drinking and going to whore houses.
That sort of thing looks good on evals, which help with promotions.
as for volunteering........well, the old Navy joke is.......
Navy stands for.......
Never
Again
Volunteer
Yourself.