The tab allows you to configure which child relationships to display in the diagram.
By default, Structur adds the object of the top record.
Click on "Add Relationship" to display the dropdown of child relationships for this object.
You can add up to 9 levels of child relationships. Be aware though that the more you add, the slower Structur will take to load.
Adding same object multiple times
If you are adding the same object multiple times (like in the example above with accounts), then you can only add child relationships to the top one. This will affect all the other ones.