[css-display] Where is <display-box> defined?

http://dev.w3.org/csswg/css-display-3/ uses <display-box> in the
definition of 'display', but does not define it, and I can't find it
defined elsewhere.

The grid thread linked below suggests that it shares some values with
'box'.  Is there some relationship between <display-box> and the 'box'
property?

http://lists.w3.org/Archives/Public/www-style/2012Nov/0459.html
> via "display-box: contents;"

cheers,
mike