Root Objects represent the set of data you want to approve.

In case you created the Design Verification workflow from the Part profile card, this Part is automatically added as Root Object of the workflow.

It is possible to add multiple Root Objects to the workflow, so that you can approve different Parts with the same Change Item. 

To do that, you simply need to access the "Root Objects" tab from the Design Verification workflow profile card, and select the following icon: 

Complete documentation: Design Verification Workflow FAQ