SCUP can retrieve all available catalogs that are free -> downloads the meta data for the catalogs you imported and saves it in WSUS -> SCUP admin publishes one or more updates (automatic, full content or metadata only) -> updates are stored in WSUS (depending on the settings from the last step). Updates (metadata) will be imported into ConfigMgr on the next SUP sync. ConfigMgr admin adds updates to an update list and “downloads” the binaries from WSUS. They will be stored in an updates packages that has to be distributed to DP(s). Clients locate content on DP (depending on the boundary/group settings etc)
With ConfigMgr 2012 SP1 there is the ability to have multiple SUP’s in a site. FOr high availability, you will need to publish the SCUP update to both SUP’s as SCUP will only by default connect to one SUP and publish updates. You will have to manually connect to the additional SUPs and publish the update content.