What's the Difference Between Layer and Mask?

When should you use Layer instead of Mask or vice versa?

Mask is used to create a card backing. It can be set to be viewed 3 ways: Back, Front, and Front with a icon graphic to show that it’s face up to the player who owns the card but still face down to their opponent.

Layer is used to change the visuals on the piece either manually or based on the value of a global or dynamic property. For example, you can have a tank counter with a blank transparent graphic for level 1 of the layer and a transparent graphic with the word “FIRED” on it for level 2 of the layer. When the tank is fired, the layer can be switched to level 2 so that now the tank has the word “FIRED” written across it.