The containing (main) article, the sub-article(s), or both, are often quite small. For example:

• An article titled “Member News Column” may contain one paragraph introducing the news items (main article) and then contain several longer news stories (sub-articles) from the members.
• A summary article which begins with a full review of the literature (main article) and includes several small related essays (sub-articles).
• Many journals have regular columns such as “News”, “Book Reviews”, or “50 years ago in the journal”, which consist of a series of small entries grouped under a single article title. Each such column could be tagged as an article with the introductory material as its direct content and the various small entries as sub-articles.

In each of these cases, the overarching usually contains a title and some metadata, may contain some introductory textual material, or may be a full article.

Another common use for <sub-article>s is to contain several language variants of the same article, where each sub-article contains the same full logical article (same content) in a different language (or languages). The <article> element serves mostly as a vessel for the multiple equivalent logical-articles, each of which is tagged as a <sub-article>. Each <sub-article> takes an @xml:lang attribute and can include full metadata (in <front-stub>), a complete body, back-matter, references, etc. The containing <article> may or may not have any content except a small amount of metadata.

A few suggestions for the multi-language use of <sub-article>:

• Both the overarching article and the sub-articles should contain the element <content-language> as part of their metadata to name the primary language(s) of the article or sub-article. For best practice, the <content-language> element should contain (as text) the two-letter ISO 639 code for the language, for example, “en” for English.
• The single-language <sub-article>s are assumed to be equivalent (same content, different language), and each can be given a DOI.
• There is no general consensus as to the value of the @xml:lang attribute on the single, overarching article that contains two or more single-language sub-articles. IETF RFC 5646 (https://tools.ietf.org/html/rfc5646) allows for a 3-letter code “mul” meaning multi-language document, but acceptance is not universal, particularly since this is a 3-letter code in the set of 2-letter language codes.

For additional information and an abbreviated example of multi-language sub-articles, see Multiple Languages

The element <front-stub> is, for a <sub-article>, the equivalent of <article-meta> for an article. A <front-stub> can contain most of the same metadata as the <article-meta>.

How much metadata should be part of a sub-article versus how much belongs to the overarching article is a business decision. Sub-articles usually inherit all journal metadata (<journal-meta>) from the parent article; however, the <sub-article> element allows for the inclusion of a full <front> matter (which contains both the journal and article metadata) for the rare instance in which the journal metadata is different from the parent article.

Sub-articles also typically inherit the article’s metadata (<article-meta>) unless explicitly overridden by using a <front-stub> element or <front> element. For example, a sub-article may begin on a different page than the primary article, may have different authors than the primary article, or may need different keywords. Typically, the goal is to keep the <front-stub> of a sub-article as small as possible, listing just the article metadata that is different from the main article, but a publisher or archive may choose to replicate most or all of the article metadata inside the sub-article for more accurate record keeping or retrieval.