The -d and -D options

[JCA]

  I wonder if someone could please explain the meaning of the
following options the screen command:

      -d -r   Reattach a session and if necessary detach it first.

      -d -R   Reattach a session and if necessary detach or  even  create  it
              first.

      -d -RR  Reattach  a  session  and if necessary detach or create it. Use
              the first session if more than one session is available.

      -D -r   Reattach a session. If necessary  detach  and  logout  remotely
              first.

      -D -R   Attach here and now. In detail this means: If a session is run-
              ning, then reattach. If necessary detach  and  logout  remotely
              first.   If  it  was not running create it and notify the user.
              This is the author's favorite.

      -D -RR  Attach here and now. Whatever that means, just do it.

I am enclosing them with their description in the screen man page
because, quite frankly, I do not understand the differences (they all
sound very similar), and under what circumstances I would want to use
one of them rather than the others. The "if necessary" qualifier is
particularly galling, for it is not at all clear (to me) how to
determine the necessity requirement. The description of -D -RR is the
very worst, for it really is a useless explanation if there has ever
been one (and there has.)

  Any help will be most welcome.