Pay stubs are a mix of requirements. Some items must be there, others depend on the company, and even a union contract. I worked for one company a long time ago where the monthly coffee club dues could be an item on your paycheck. Of course every company uses different codes and abbreviations.
HR may know, if I was to contact somebody besides my boss then I would start with HR.
Another way to find out is to look at all the pay stubs and determine when this first appeared. That may give a clue as to the purpose.
You said that this year you have been "furloughed on and off", are there hours or $s with that category during the weeks you were furloughed? Or are they only on weeks where you took vacation, or comp time, or some other category of hours?