Agilent Technologies ENA Series User Manual page 167

Fixture Simulator
Evaluating Balanced Devices (balance-unbalance conversion function)
Figure 7-7 Type of balanced devices that can be evaluated with E5070A/E5071A
In the terminology of the E5070A/E5071A, ports after the balance conversion are called
logical ports (or DUT ports).
You can assign the test ports of the E5070A/E5071A to logical ports (a to d in Figure 7-7)
freely.
Chapter 7 159