-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 running, 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.