Every hand you make in PLO uses exactly 2 cards from your hand and exactly 3 cards from the board. The split is always 2 and 3.
Why this is different from Hold'em
You are dealt 4 hole cards in PLO, but only 2 of them ever count toward your final hand. The other 2 sit out, no matter how good they look. On the board side, all 5 cards get dealt, but only 3 of them count.
In Texas Hold'em you can use any combination of your hole cards and the board, down to playing the board itself with none of your own cards. PLO removes that freedom. The 2-and-3 split changes which hands are actually strong: a hand that looks dominant if you could pick freely from all 9 cards can fall apart once you remember half your hole cards are locked out.
Two examples that catch new players
Hold two Aces and see one Ace land on an otherwise unpaired board. Playing your Aces makes three of a kind. You use both hole Aces plus the board's one Ace for three of a kind. The board has no pair to build a full house, and no fourth Ace anywhere on the table to build quads.
Quads are still possible in PLO, but not from a single Ace on the board. Picture the board itself showing three cards of the same rank, say three Aces. If you hold one more Ace plus any other card, playing that Ace and the other card as your 2 hole cards against the board's 3 Aces makes four of a kind, with your other card as the kicker.
A hand open in the Hand replayer: the table between the hand's context bar and the street tabs, with the analysis panel beside it.
Keep the 2-and-3 split in mind whenever you compare your hand to the board: in The Hand Replayer or while studying a range in Strategies.