Transclusion is pulling content dynamically from one page into another page. Rather than cutting and pasting text from one page to another, you create a pointer to the page you are borrowing from. That pointer is resolved at run time, pulling content from the other page when your page is loaded. Transclusion was a fundamental part of Ted Nelson’s original concept of hypertext. It has never caught on, except in specific confined circumstances. Despite continued interest, it isn’t going to catch on.
One of the most important distinctions we need to make when writing for the Web is the difference between how texts are related and how subjects are related. If that sentence made you say “Huh?”, let me explain.
In the book world, the principal reason for one book to refer to another book or article was for purposes of citation. A citation says, for support for this claim, see this work. The citation is used either to prove an assertion about another text, or to support an assertion about a subject with reference to an authoritative text. Any old text on the subject won’t do. It is the authority of the specific cited text that is being invoked.