With this checked, SpamSieve will quit itself when you quit an e-mail program, provided that there are no other mail programs running. This prevents SpamSieve’s icon from filling up your Dock when it’s not actually being used (unless you have Show recent applications in Dock checked in the Dock pane of System Settings). Also, it prevents SpamSieve from relaunching Mail or Outlook if it’s in the middle of filtering when that application quit.