What does “remove” mean?

[David Abrahams <dave@boostpro.com>]

So I'm looking at this help:

  wl-dispose-folder-alist is a variable defined in `wl-vars.el'.
  Its value is 
  (("^-" . remove)
   ("^@" . remove))


  Documentation:
  *Alist of folder and dispose policy.
  Each element is (folder-regexp . policy).

  The policy is one of the followings:
  'remove or
  'null     : remove message.
  string    : refile to the specified folder.
  'trash or
  otherwise : refile to the `wl-trash-folder'.
  ex.
  '(("^%" . "%#mh/trash")
    ("^-" . remove)
    ("^\\+" . trash))

I know what “refile” means.  What does “remove” mean?  Especially, what
does it mean in the case where the pattern matches "^-", i.e., an NNTP
folder.  And what does it mean in the case of IMAP, where messages can
be deleted or expunged?

Thanks,

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com