Vertex attributes in this mesh will override or add attributes of the primary mesh and the additionalVertexStreams in the MeshRenderer.
This vertex stream is used exclusively for the UVs for realtime lightmaps.
The realtime lightmap UVs from the primary mesh and the additionalVertexStreams are overridden by this stream. Not serialized. This stream is set by Unity when reatime GI precomputation is done and when loading a scene with precomputed realtime GI.