Decals are 3D objects that are separate from the ordinary six-sided segment geometry in a level. Two types of these objects can be added to a single segment face, and rotated/aligned/repeated as needed to add visual improvements or clarity to a level. Some decals are emissive, meaning they project a (by default) shadow-casting light source. By default, all decals will collide with the player and robots, so some very large decals that obstruct the geometric center of a segment will prevent robots from navigating through that segment properly. It is advisable to mark said segments as Pathfinding: None in order to prevent any problems arising during gameplay.
New decals can be created using the DMesh Editor, included with every installation of the Overload Level Editor.
Decal Removal
Decals can be added to a face, but not deleted per se. The closest analogue to true deletion is only to hide them, though adding segments will propagate some face decals outward into the new segments. Hiding one or both decals from a segment face prevents them from rendering, colliding, or even exporting in a level. Should a creator wish to un-hide a decal, all of its pre-deletion settings are used.
Fisholith has created a decal named '!REMOVE' that decals can be reassigned to in order to quickly find and hide them in groups. Due to its naming convention, it will always be found at the top of the decal list. It can be found at this dropbox link.
Detailing usage of Fisholith's 'Dedicated 'Remove' Decal Mesh'
Parabolicus has made a Python script that imports .overload levels and specifically strips decals marked as hidden. It can be found at this link (Discord).