Model Data is the data for the site on the tabs:
and data for building model on the tabs:
The Model Data tabs are shown in the Edit Screen - User Interface help topic.
At the point of the building is created, building model default data is initially loaded at the building level from templates. This is inherited from the building to blocks to zones to surfaces and to openings. This default model data can be overwritten by making edits on the Model Data tabs on the Edit screen.
See the DesignBuilder User Interface – Model Data: Site level, model data tabs, data hierarchy and inheritance tutorial