Mention how to select just those headers shown, but without the numbers

(info "(mailutils) Specifying Messages") says
     ':n'
          Selects all recent messages, i.e.  the messages that have not
          been neither read not seen so far.
                              ^nor

Also please mention or link to what the difference between read and seen
is.

Also please mention, how when one is scrolling in pages of 20 messages,
how to say "delete all these shown", without the numbers.
d 1-20
h
d 21-40
h

but an easier way.
No
d * is overkill.

I.e., what MSGLIST refers to just the 20 currently shown, but without
using numbers.