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, normed spaces, ... At the moment there are 53 questions having this tag. Majority seems to be about subspaces of vector spaces.
It seems that we have basically two options:
- Use this tag as a kind of "meta tag" - the meaning will be clear from other tags added.
- Specify for which of the possible meanings it should be used. (Although in that case it would probably be good to rename tag to something more descriptive.)
I will mention that some similar tags have been discussed -- I mean various tags related to creating new structures from old ones, where the particular construction is applicable to various types of structures.
- Creating products-abstract for products of various types of structures was proposed here: What should be in (products) tag?
- Tag quotient-spaces was discussed here: What is the tag (quotient-spaces) intended for?
arewere tags direct-limit and inverse-limits; see here: Tag management 2015 (It seems that the later has been removed shortly after I made this post: 1, 2, 3, 4. The former was removed a bit later: 1, 2, 3, 4, 5, 6, 7, 8.)
- There exists also the tag limits-colimits, created after the discussion here. However, it seems to have category-theoretic flavour, so using it for products, quotient spaces, subspaces and similar stuff seems like a stretch if there is no category theory involved.
1In the meantime, the tag-info has been added. The current wording seems to suggest that the tag is for vector subspaces. (I will also add link to the revision history of the tag-excerpt and the same for the tag-wiki.)