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To explain I mean to be able to make a selection set a mesh light by using the tag instead of having to separate that part of the mesh and applying the tag separate there should be something like a slot that asks for selection tags if needed. I'll explain more if you don't get it thank you.
Currently Arnold does not accept polygon selections as a source of a mesh light, only complete meshes. Yet some custom export logic could be possible, which basically extracts the selection as a separate mesh. I'll log this in a ticket. Thanks.