In its essence, a Decal is an image that is projected onto a surface. In-game, this usually this means a logo or text on a building.
<Insert image example of decal>
A Decal consists of four 'layers', which are defined in the ARGB color channels of the RASTER image, with the Alpha channel being the top layer and the Blue channel being the bottom.
Decals are defined in a Decal Dictionary (also referred to as Decal Atlas in some instances) that contains a table of specific values relevant to a single decal, such as the RASTER image to use, the colors for each layer and the aspect ratio of the Decal.