The optional second parameter is Unity layer you want the asset to be associated with. So now Unity has a reference to this sprite sheet: Next we need to define an animation to play. We can do this ...

Learn how to use sprite sheets, which are images that contain multiple sprites or frames of animation, to optimize your 2D graphics rendering performance and create smooth animations.

These include the 2D PSD Importer that allows layered Photoshop images to be imported directly into Unity. 2D Animation provides advanced tooling for 2D skeletal animations from within the sprite ...