[css3-flexbox] 'display:box' is not defined

The spec relies on a new value for 'display' property - 'box'. The value is not defined in the spec however, although it is used in examples, and it is apparently required in Mozilla and Webkit implementations. Shouldn't the new 'display' value be defined in the same spec?

BTW I am not sure how intuitive the term 'box' is here. I've seen it called 'stack' in other similar concepts. For me a 'stack' would be easier to discover...