A unique index, used internally for identifying lights contributing to lightmaps and/or light probes.
When manually assembling a lightmapped scene set this index appropriately, so that the light doesn't affect lightmapped objects twice, but at the same time affects dynamic objects. Having a unique index for each light helps to mix realtime and baked shadow by identifying the amount of light contribution each particular light had on a lightmap or lightprobe. -2 means that light is not baked, -1 is reserved for internal use. See Also: Light component and isBaked.