There are 3 different phrases associated with gaining cards, all of which have different meanings.

GAIN A DRAGON OR A CAVE CARD: Unless additional restrictions are specified, you may gain a card from either the face-up display or from the top of the deck. Note that there may be times when you have a “gain” ability, but there are no face-up cards in the display. If this happens, you may still draw from the top of the deck.

GAIN A DRAGON OR A CAVE CARD FROM THE DISPLAY: If the “from the display” instruction is included, you must select a face-up card. If there is no card in the display, then you may not take this benefit.

DRAW A DRAGON OR A CAVE CARD: To “draw” means you may only take a card from the top of the relevant deck.

REVEAL A DRAGON CARD: Show the top card of the deck to all players. The ability will specify if you do something with the card; if not, return it to the top of the deck.