What is diffrence between Load Base Data and Load Demo Data?

Based Data - this is mandatory data set required for all Windchill solutions. This includes all the out of the box templates - LCs, WFs, OIRs and all. This is required.

Demo data - This is a set of demo data includes a couple of test Windchill products, some demo CAD files loaded in a demo organization. This is not something which you load in a production system.

There really should be a little note on this right in the installer UI that warns you not to check yes for the demo data for your production system install.