Use the following steps to define a differential pair:
Create a new Object List entitled Differential_Pairs.
Select Object Type as Diff Pair in the Properties window of the Differential_Pairs Object List.
Set Contents = ‘Manual’ or ‘Auto’. If you choose Auto, specify the criteria to auto-populate the Object List with differential pairs. For ‘Manual,’ select Diff Pair >> hold Shift then left-click to get your grouping of diff pairs >> select the (>) arrow to move them into the Object List. After hitting > arrow, you will see: