extra.* Entities

I've been going through the new HTML-in-XML working draft. Now I have a
question:

Can someone explain to me the need or purpose of the extra.inline and
extra.block entities?  I read the note ate the top of
http://www.w3.org/TR/WD-html-in-xml/dtd.html, but I guess I don't know
when/why you would add new entities. Is it for later versions of HTML (post
v5)? Or is it something used so that tags from other DTDs can be laid out
correctly in an HTML flow? Can someone provide an example?

I've never been a big SGML or DTD hacker so I assume I'm missing some unsaid
convention that is supposed to make life easier in the long run.

Andrew n marshall
  student - artist - programmer
    "Everyone a mentor, Everyone a pupil"