Your cap pouch attaches to your waistbelt and contains the percussion caps for your musket. There are lots and lots of different styles for cap pouches, depending on the contractor that made them and the time that they were manufactured during the war. Most come with lambswool just inside the flap to keep the caps from tumbling out if the cover is open and you are running or loading from the prone position. There is also, sometimes, a place to store a nipple pick.
Some pouches have the name of the contractor stamped on the leather cover flap.
If you plan a Federal impression, get an M1850 cap pouch in black leather.