A tag segments a newsletter's audience. Sending an article to a tag stamps
that article with an audience, which is what makes it invisible to everyone
outside it. Named for the subscribers it is applied to, because a tag
called Tags inside a document made of tags says nothing.
Applying and removing a tag are writes, and they are grants and revocations of access to whatever articles that segment was addressed to, not only labels. Creating, renaming and retiring a tag are not operations here yet.