Apply a tag to a subscriber

Puts this subscriber in this segment. Needs audience: write.

One subscriber per request. To tag a list of them, up to 500 at a time, use Apply a tag to many subscribers (POST /tags/{tag}/subscribers), which costs one request however many it names and reports what became of each.

A tag decides what a person can read, not only who receives what. An article addressed to a tag is readable by the people holding it and by nobody else, on the web as well as in the inbox. Applying one grants access to every article that segment was ever addressed to, including articles sent before this call. Removing one revokes that access immediately.

Applying a tag the subscriber already holds succeeds, changes nothing and publishes no event.

A retired tag answers 422. A retirement stops a segment gaining members while it goes on deciding who may read the articles it was addressed to. Removing a retired tag still works, since taking a tag off only ever narrows what somebody can read.

The response is the tag, not the subscriber, so this operation never puts an email address in a response.

Publishes subscriber.tagged with direction: assigned, unless the subscriber already held the tag.