Baked lightmap UVs are per-mesh: all instances of the same mesh share the same baked lightmap UVs. Unity can generate these UVs when you import a model, or you can provide your own data.
Unity stores baked lightmap UVs in the mesh in the Mesh.uv2 channel. This channel maps to the TEXCOORD1 shader semantic and is commonly called UV1.
When you enable Baked Global Illumination and a MeshRenderer receives its global illumination from lightmaps, Unity uses the data in the Mesh.uv2 channel to correctly map the baked lightmaps to the mesh.
Note: If you want to use Mesh.uv2 for another purpose in a given mesh, you must ensure that all MeshRenderer components that use the mesh receive global illumination from light probes rather than lightmaps. Configure this in the Mesh Renderer component Inspector, or via the MeshRenderer.receiveGI API.
For more information, refer to Generating lightmap UVs.