An additional layer that magnifies part of the illustration. It's a common technique used for backgrounds in character art.
There are sometimes small, primarily 2D variations, such as:
The layer being reversed.
The layer being rotated.
The layer being partially transparent, in greyscale, or having other changes in color.
A character having a different expression.
If the layer is from a different 3D perspective, or there are larger 3D differences such as costumes, poses, or characters, it's a projected inset instead.
