When I view local options (i.e. :setlocal) - what do the "odd" prefixes (i.e. '--') mean?

Question

I'm working with CentOS 7, and Vim 7.4,

:setl
--- Local option values ---
  autoindent          define=             include=            path=               softtabstop=4
--autoread            errorformat=        keywordprg=         readonly            syntax=cpp
  cindent             filetype=cpp        makeprg=            shiftwidth=4        tabstop=4
  cryptmethod=        grepprg=            number              smartindent         tags=
  comments=sO:* -,mO:*  ,exO:*/,s1:/*,mb:*,ex:*/,://
  fileencoding=utf-8
  formatoptions=croql
  omnifunc=ccomplete#Complete
  undolevels=-123456
:version
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Jun 10 2014 06:55:55)
Included patches: 1-160
Modified by <bugzilla@redhat.com>
Compiled by <bugzilla@redhat.com>

My main question:

What does the -- prefix in front of the 'autoread' option mean?

Next question:

Since I could find no help on this anywhere, are there any other prefixes that I should know about?

Answer 1

From :h :setlocal:

                                                        :setl :setlocal
:setl[ocal] ...         Like ":set" but set only the value local to the
                        current buffer or window.  Not all options have a
                        local value.  If the option does not have a local
                        value the global value is set.
                        With the "all" argument: display local values for all
                        local options.
                        Without argument: Display local values for all local
                        options which are different from the default.
                        When displaying a specific local option, show the
                        local value.  For a global/local boolean option, when
                        the global value is being used, "--" is displayed
                        before the option name.