A <sub-article> is an article of any type (letter, reply, review, short new piece) which is entirely contained within another article.
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 three longer news stories (sub-articles). An example of a longer article is 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. The group as a whole might have some introductory material in addition to the title. Each of these columns could be tagged as an article with the introductory material as its direct content and the various entries as sub-articles.
How much metadata is part of a sub-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> (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 need its own article metadata because it begins on a different page than the primary article, it has different authors than the primary article, or it takes different keywords. Usually 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.
The decision of whether one article should be bundled into another, and thus tagged as a sub-article rather than a full article in its own right, is subjective. Publishers and archives may wish to establish house guidelines for such decisions.