A main goal of PreTeXt is to provide a language for describing a scholarly document by its structure, with contained content, and with no description of the presentation. It then becomes possible to use software to produce different formats, where the presentation takes advantage of that format and enhances the meaning of the content through the expression of the structure.
But different output formats have different capabilities. For example, a conversion to HTML can take advantage of knowls to organize smaller chunks of content, while a conversion to PDF can take advantage of page numbers for cross-references. And in these two examples, the capability of the one output format is mostly impossible or silly in the other. Look here for notes that are independent of the PreTeXt vocabulary, and specific to the format produced by a conversion.