The PreTeXt Guide

The @component attribute is used by an author to identify elements in the PreTeXt source. The value of the attribute is any name that makes sense to a publisher. Examples might be videos, labs, genome, or color. There is a (huge) un-component which is the collection of all elements not contained in an element that is in some named component. You can think of it as a default component, or an un-named component—it is content that will be in every version, no matter what.