Actions for Unassigned Domains

There are three options when you click the three dots on the unassigned domain:

Make Parent 
This will move the domain to the Assigned Domains panel.
Make Child 
This will make the domain a child. You can only use this option when there is an existing domain in the Assigned Domain panel, which must be selected in the wizard.
Assign Children 
This is useful when you want to assign children in a single step. The parent must exist in the Assigned Domain panel.