A differential lumped port has explicit positive and negative terminals. To define a differential port, assign a port attribute to a face or edge on a conductor in your system, and set the Type to Lumped. Then assign a second port attribute to a face or edge on a different conductor, again set the Type to Lumped, and change the Number of this second port attribute to be the negative of the Number of the first port attribute.
For a differential lumped port, the positive and negative terminals define the two ends of the voltage differential. As a result, port patches are not necessary.
As you can see from this example, the terminals need not be coplanar. There is no restriction on the placement of these terminals within the structure: they may be interior or exterior (as shown).
Your differential port may have multiple negative terminals, as described in Port Indices. The following figure shows a coplanar line with two negative terminals.
For more details on sourcing coplanar lines, see Coplanar Lines.