I can't understand the point of "mark as read".
Where "read" presumably means 'past tense of read' ?
So the headers or [?what] are to be marked as if
they have already been read[ed] ?

Is this so that when you subscribe [or first time read the servers
list of available groups] and group G has a million articles stored
back to now - 3 years; and you just want to get the latest 50;
you can 'pretend that you've "read" all the existing ones.
And next time on line you just get the 'new' ones ?

If not how do you:
1. get the latest 50 headers of a specific group ?
2. jump directly to the bodies which where last downloaded,
by previously being 'marked for downloading, and which
have not yet been read ?