The decal layer lies on top of the background layer. It can be used to add detail to this background layer. The biggest difference between the detail layer that is a part of the detailed background or the randomizer background and a decal layer, is that the latter one can be targeted by triggers or behaviours of entities.
Example : an arrow image is used as a decal to point to a seemingly empty slot. The empty slot has a trigger, that spawns an item and removes the arrow decal from the other slot and then removes the trigger itself, all when the player enters this slot. If the player ignores the arrow, he might miss out on some loot. Once the trigger has done its job, it's removed as well as the arrow pointing to it as it no longer serves a purpose.
All sprites are available for use in the background layer. The hue, opacity, brightness, vertical and horizontal flip and rotation can be adjusted.
For info about the panels in the editor which allow you to edit decal layers, visit this page.