A layout overlay is the top level of non-animated artwork, designed to create depth. Specifically, an overlay is the element that must be separated from the background and placed at the very top of the artwork pile, to allow a character to move freely on stage. The diagram below illustrates a background level, animated character level and the overlay of a tree.

An overlay/underlay is a level of non-animated artwork, designed to assist in creating depth. An overlay/underlay is planned separation of more elements from the background, as required by the animation in the scene, than an overlay.

Some studios describe an UL as any layout artwork under the animation, and OL as the layout art on top of the animation.

Field Guide Level Sketch (left). Layout Level Separation: BG, ANIM1, OL/UL, ANIM2 and OL.

Held Cels (HC)
Held cels are separate elements, which at some point in time, will be animated. Objects and characters that are required to remain stationary for a period of time before moving, during a scene or sequence of scenes, are labeled as a Held Cel. Props such as a chair, a door, a car, a glass and a cookie often move.

Unlike an overlay or overlay/underlay, a held cel is ordinarily not part of the background location design. It may be drawn on to a location design for reference and size relationship, but will always be depicted on a Prop Sheet.