The company I'm working on has a RUP-like software development process, with very heavy documentation. Right now we're doing a Multi-Channel web site -it's intended to be seen in Desktop, Tablets and Smartphones- and every version of the site has a particular flow of events and content.
I don't see in RUP an artifact were this differences are documented. I'm aware that Uses Cases must be implementation agnostic -we can't mention any device specific terms- so maybe there's something in the Design discipline that serves that purpose.
If there's nothing like that in RUP, It would be nice to know how another Software Process document requirements for Multi-Channel sites.