It has two serial port interfaces, but only one serial device.
The Standard DB-9 port and the PWM port TX, RX, GND.
Of course IFI says you shouldn't use the PWM connector. They didn't say you can't however.
http://www.ifirobotics.com/forum/viewtopic.php?t=165
The serial port is used to send info about which breakers are tripped to the RC. The default MPLab code already has support for that data.