There is a tag called subspaces. (It seems that it was created by this user.) Both tag-wiki and tag-excerpt are currently empty, so the usage is unclear.1 If we consider just the name of the tag, there are plenty of situations where it can be used: vector spaces, topological spaces, metric spaces...

Update. The subspaces tag has been removed thanks to a friendly neighbourhood Community Manager. There's another possibility that hasn't been mentioned in the OP, and the one I would prefer: kill it! I don't see subspaces as a tag that will ever be consistently used, and not something that ...