Decals versus overlays
Individual decals are relatively inexpensive to render and can be used to add detail, break repetition, and create effects you cannot with standard textured surfaces alone. Decals however have a limitation in the fact they are always the same size and you have no control over orientation as it is decided by the face underneath the decal. Overlays allow those features but have a (slightly) higher cost to render. For most cases, overlays should be used instead of decals. The main advantage of decals is their ability to be enabled and disabled via inputs.