In the US, Regular Army is RA.

Elvis was conscripted into the regular Army.

He was in the Regular Army--3rd Armored Division in Germany.

A national army is a regular army. If, however, you want to know the difference between the National Guard and the regular Army, the National Guard is a part time reserve drawn from the general population who are mobilised in periods of national emergency.

The same as for Regular Army. As a deployed NG service member, you are Army, with all the benefits and perks of Regular Army. You also serve the same as Regular Army, and that usually means in any manner as needed and dictated by command.