Your understanding is Right ! So no any questions remain to answer...
"Upload data" is something that's currently limited to the latest releases of BIP and the upcoming cloud version of OBIEE. However, the could version will not allow mash-ups since what you upload is what you model. There is no "central, IT-managed RPD" in the on-premise install sense. This will evolve obviously as the versions progress.
BIP uploads on the other hand provide - at the moment - to load your own data and the report on it. Downside: this covers XLS sources and you can't mash up data with OBIEE- / RPD-sourced information.And especially not access the metadata layer to hook up with an uploaded XLS.