Each port must have an integer index, referred to as the port number. These indices are used to define the relationships that you want to maintain between the ports, according to the following rules:
All ports with the same index magnitude are part of the same port. Positive index attributes are excited to a positive voltage (relative to ground), and negative index attributes are excited to a negative voltage. Thus for common mode excitation of two traces, the port attributes assigned to the trace ends should both have the same port index and sign, e.g., 1 and 1. You can obtain a differential mode excitation by changing the index of one of the port attributes to the negative of the port number, so the indices become 1 and -1.
The collection of ports with indices of the same magnitude but differing signs is described as a differential port, and each individual port assignment is a terminal of that port. When you define a differential port, that port may have at most one positive index terminal, and any number of negative index terminals.
You may define any number of ports with different indices. Ports with indices of different magnitudes are treated as independent sources of excitation.
In a structure with multiple ports, the port index magnitudes must increase consecutively starting at 1. A simulation with a standard port of index 1 and a differential port of index 2 will have three port attributes, with indices 1, 2, and -2.