Throughout Rewriting: How To Do Things With Texts (Utah State University Press, 2006) Joseph Harris presents highlighted sidebar presentations he labels "Intertexts".
They simultaneously serve the functions of footnotes, references, (pseudo-)pull quotes, and conversation with his own text. It's not frequently seen this way, but these intertexts serve the function of presenting his annotations of his own text to model these sorts of annotations and intertextuality which he hopes the reader (student) to be able to perform themselves. He explicitly places them in a visually forward position within the text rather than hiding them in the pages' footnotes or end notes where the audience he is addressing can't possibly miss them. In fact, the reader will be drawn to them above other parts of the text when doing a cursory flip through the book upon picking it up, a fact that underlines their importance in his book's thesis.
This really is a fantastic example of the marriage of form and function as well as modelling behavior.
cc: @remikalir