Adding a Data Definition Group can be done by dragging the Data Definition member from the Data Definition pane to the Content Wizard pane. You can add Data Definition Groups to the Content Wizard element, to other Data Definition Groups, or to Sections.
You can change the order of Data Definition Groups of the same level by dragging the elements or by using the arrow buttons in the toolbar. Data Definition Groups from different levels have to maintain their relative ordering though.
Example Data Definition:
Customer
Partner
PartnerProducts
PartnerChildren
Products
Children
Based on this example, you can add Data Definition Groups to the Content Wizard based on Partner, Products, and Children and change the order.
For instance, first add a Data Definition Group Children to the Content Wizard and then add another Data Definition Group Product on the same level. Then, you cannot add PartnerProduct without first adding a Data Definition Group Partner and adding the PartnerProducts to that Partner. It is also not possible to add Products to Partner or to add Products to Children.
Sections and Selection Groups have no influence on this order. For instance, you can add a Data Definition Group based on Products and add on the same level a Section and add a Data Definition Group Children to that Section.