Transclusion is one of the foundational concepts of DITA. Coined by hypertext pioneer Ted Nelson, the term "transclusion" refers to the inclusion of part or all of an electronic document into one or more other documents by hypertext reference.
"Transclusion allows content to be reused far more efficiently."
The concept of transclusion took form in Mr. Nelson's 1965 description of hypertext. However, widespread understanding of transclusion was limited by the slow adoption of markup languages, including Structured Generalized Markup Language (whose origins date to the 1960's), Hypertext Markup Language (released in 1993), and eXtensible Markup Language (released in 1996). In fact, it wasn't until DITA, an XML vocabulary donated to the open-source community in 2004 by IBM, that the power of transclusion enjoyed broader reception.
Transclusion differs from traditional referencing. According to The Content Wrangler's Eliot Kimber, traditional content had "…to be reused through the problematic copy-and-paste method." With transclusion, a hyperlink inserts content by reference at the point where the hyperlink is placed. Robert Glushko adds, "Transclusion is usually performed when the referencing document is displayed, and is normally automatic and transparent to the end user." In other words, the result of transclusion appears to be a single integrated document, although its parts were assembled on-the-fly from various separate sources.
"In the information management sense, transclusion makes content easy to track, removes redundant information, eliminates errors, and so on," writes Kimber. "Use-by-reference serves the creators and managers of content by allowing a single instance to be used in multiple places and by maintaining an explicit link between the reused content and all of the places it is used, which supports better tracking and management."
Transclusion is not without its limitations. It's rarely used in web pages, where the processing of transclusion links can become cumbersome or can fail when the page is displayed. For that reason, people writing content for the Web "…do the processing in the authoring environment and deliver the HTML content with the references already resolved. However, transclusion, which doesn't rely directly on metadata is superior to conditional preprocessing when working with content that has a large number of variations.