We have by chance found out that you can send by core service keyword to publish. Once in queue, you can even see it from GUI. We know that's not allowed from GUI (only category can be published). SO my question is does anything special happens once keyword is published, or resolver just clears it all out?
We observed that if you publish keyword, it will publish only keyword. Publishing category will publish full taxonomy.
Content Manager always publishes full Taxonomies (a Category and all its Keywords).
If you publish a Keyword, it resolves the Taxonomies (Categories) it is part of (that's normally exactly one) and those get published.