“what does “Special Duty” mean?”
Jobs that suck (more than most military jobs), or are more hazardous for one or many reasons.
This is just a way to prevent people from leaving when their enlistment is up.
These are not all combat related, they’ve been around for a long time. We had them when I was in the USAF in the ‘80’s.
My Father got special duty pay in WWII. He was in the Army Quartermaster Corps in the Pacific.
He was a diver, the hard helmet type, who primarily worked at clearing wreckage and prepping piers in recently taken harbors. He mentioned it as ‘hazardous duty pay.’