The layer in front of the background layer.

You see the elements of both layers when you look at a card as if the card layer were a transparent layer in front of the background layer.

The card button, card field, card picture, card portal, or card shape created most recently is ordinarily the topmost object in the card layer (that is, closest within the card layer to the front of the screen). All elements in the card layer are in front of elements in the background layer.