In UNIX, what are the Ss, Sl, and Ssl proccess types I see with ps aux?

From the ps manpage:

   S    Interruptible sleep (waiting for an event to complete)

  For BSD formats and when the stat keyword is used, additional
  characters may be displayed:
  s    is a session leader
  l    is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)