Most control types have one or more "Layer" arrays. For many, it is simply called "Layers". For others, there may be separate "layer" arrays for each component of the control (such as the filled, empty, and knob layers of a Slider control). These arrays are essentially arrays of references to other sprite objects which can be used to serve as additional visual "layers" that appear on top of the visual elements that make up a control. These layers can be used to add additional visual effects or animations to the control.

Sprites in a layer will change along with the state of the associated control. For instance, you might want to have a pulsing glow animation when a button control is in the "over" state, and a television static-like animation when the button is disabled. To achieve this and make sure that the sprite changes along with the control, you would create two animations on the sprite - one called "over" and one called "disabled". You then add an element to the control's respective layer array, and then drag the sprite onto the new array element.

Positioning
When using layers, it is up to you to make sure the other sprites that make up the layers are positioned correctly over the desired visual element. It is also a good idea to make the overlay sprites children of the desired control object so that they will automatically move with the control when it moves.