public Texture holesTexture { get; }

An uncompressed Terrain holes Texture is a RenderTexture in the Editor and at runtime. A compressed Terrain holes Texture is a Texture2D at runtime only. You might use an uncompressed Terrain holes Texture as a render target for the custom painting of Terrain holes data. Use the _enableHolesTextureCompression property to enable or disable Terrain holes Texture compression at runtime.