The PC Compatibility Card will actually use the first Ethernet interface it finds. On systems with built-in Ethernet, this is always the built-in Ethernet. On systems without built-in Ethernet, the PC Compatibility Card will use an add-in Ethernet card. For example, the PC Compatibility Card will work with the Comm Slot II Ethernet Card in the Power Macintosh 4400/200.
There is no way to select an Ethernet interface like you can in the AppleTalk control panel. Therefore, on systems with built-in Ethernet, the built-in Ethernet is always used.