updates for Numbers, Pages, and Keynote say that you can do collaborative work using Box. What do we know about this "Box"? Searching such a short word on Apple's site seems hopeless.
This refers to Box.com, a third-party collaboration tool with a focus on file sharing, similar to Dropbox.
As for why this company specifically: it is known that Apple internally uses Box for file sharing, so they have an interest in integrating this platform to assist in their own use of their iWork suite.