Journals frequently contain correspondence, often in response to previously-published articles or to earlier correspondence. This Tag Set has no explicit “letter” element or “correspondence” element. An item of correspondence may be adequately tagged as a standalone “article” (<article>), with the @article-type set to:

• “letter” for letters that respond to a full-fledged article, or otherwise initiate a topic of correspondence;
• “reply” for letters that respond to other letters (one common case is where the author of an article responds to correspondence about that article).

When a series of responses is published in the same issue along with the original article, the responses can be tagged inside the original article. A journal might choose to consider these as responses to the original article (<response>) or as related subordinate articles (<sub-article>), and tag them accordingly. The choice is a subjective, editorial decision.

Ideally, each item in a series of correspondence will link to the item it responds to, and to (at least) the first item that responds to it. If the item referred to is an article, this can be done with the <related-article> element. If the referring and referred items are both in the same article (as when one or both is a <response> or a <sub-article>), this linking can be done with the <xref> element. If the referred item is a sub-article within a different article, neither <related-article> nor <xref> suffices, and <related-object> should be used instead.

In some settings it may be difficult to add links to already-published material, so that they may point forward to later responses. In such cases, it is still important to have the later responses link back to the material to which they are responding.