Sometimes, you may not want to see certain layers of certain panels in the Onion Skin. In order to exclude layers from the Onion Skin, you can set those layers as Background layers.
When you create a panel, it contains two layers: A BG layer and an A layer. The BG layer is already set to be a background layer by default, which means it will not be included in the Onion Skin. At any time, you can choose which layers in a panel are actually considered background layers or not. All layers that are set to be background layers will be excluded from the Onion Skin.
NOTE To set layers as background layers only affects whether or not they appear in the Onion Skin. Hence, setting layers that are not really background drawings as background layers will have no adverse effect on your storyboard.