The mask of heightmap layers this object uses.
These heightmap layers store actual terrain data:
Missing data is represented with the HeightmapLayer.Coverage layer. It describes how much information is present in a stored heightmap sample. The semantics of coverage with heightmap samples is the same as premultiplied alpha with texture samples. A LayerMask without HeightmapLayer.Coverage means that all samples are fully solid. If a LayerMask contains the HeightmapLayer.Coverage layer but no others, the coverage values are used to represent 'holes' in the terrain (e.g. creating holes for caves, tunnels, etc.).