What does "Reg Wages NP CP" mean on a paycheck?

There is very little standard terminology, so it probably means whatever the respective accountant in your company meant when he came up with the abbreviation.
Paychecks have typically all kinds of abbreviations, and although you can guess some, many are internal expressions from the respective HR team - and often different in every company.

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?