PCU = "Played Card Unit" - This is a card that was removed from the library during a game, i.e. the player had the opportunity to play it. Stacks (if 3x are removed from the library, PCU for that game is 3)

GCU = "Game Card Unit" - This is a card that was brought to a game. Stacks (if 4x are included in a decklist for a game, the GCU for that game is 4x)

DCU (maybe?) = "Deck Card Unit" - This is a card that was brought to a game, with Stacking ignored (if 4x are included in a decklist for a game, the DCU for that game is 1)