The note at the end of the of the element description for <back> is not at all a useful note, and it raises more questions than it answers: "The content model of back matter is identical to that of front matter, reflecting the facts of cultural history." I don't think it contributes usefully to the element description and should either be expanded or (probably preferably) deleted.</back>
This note appears to be a shortened and confusing version of what appears in the prose of 4.7:
This dates back at least as far as P4, and so does the note. I agree that it's unhelpful as it is, but it is worth making the point that back and front have a close relationship and identical content models. I vote that the note be expanded to make it comprehensible, rather than deleted.
Yes, the 4.7 note is much clearer, and I agree that with a note more along the lines of the 4.7 version, the information is quite useful--so, I change my thinking from preferring deletion to preferring a richer note along these lines.
Addressed in revision 13123.