UAC credits are only found in Doom RPG and Doom II RPG and are used in vending machines found in the game. They are either silver or gold in color, with the silver ones valued at one credit and the gold ones being five credits. They can be used to buy medikits, ammo and upgrade the marine's stats.
"Credits" are also referred to throughout Doom 3, apparently being the in-universe future currency.
The UAC credits were likely inspired by the Blake Stone coins.