The index of the lightmap applied to this renderer.
The index refers to lightmaps array in LightmapSettings class. A value of 255 means no lightmap has been assigned, which is the default. A value of 254 is internally used for objects, that have their scale in lightmap set to 0, so behave as lightmapped, but don't have a lightmap assigned. The index can't be larger than 253.
A scene can have several lightmaps stored in it, and Renderer can use one of those lightmaps. This makes it possible to use the same material on multiple objects, while each object can refer to a different lightmap or different portion of the same lightmap.
See Also: LightmapSettings class, lightmapTilingOffset property, ShaderLab properties.