X-Plane® Decals are overlay textures that sit on top of a regular day-time texture to add high-frequency, high-res detail to an existing texture.
OpenSceneryX currently only has a single Decal, which can be used to add snowy detail in winter conditions. To use OpenSceneryX decals in your own objects, use the DECAL_LIB command, for example:
DECAL_LIB opensceneryx/decals/snow/1.dcl
For more information on Decals, see the official X-Plane® documentation here and here.