TAG MIL-BOOK 10 - Manual (Page 66)

Allows you to assign COM1 and COM2 to specific functions that you
wish to use. In general, COM1 can be assigned to RS-232C (serial port)
while COM2 can be assigned to IR (infrared). Select Disabled if you need
the resources (3F8/IRQ4 of COM1 and 2F8/IRQ3 of COM2) for other
devices.
COM Port (COM3/4)
Allows you to assign COM3 and COM4 to specific functions that you
wish to use. In general, COM3 can be assigned to RS-232C (serial port) or
touchscreen, while COM4 can be assigned to RS-232C. Select Disabled if
you need the resources (3E8/IRQ5 of COM3 and 2E8/IRQ7 of COM4)
for other devices.
Setup Configuration Utility (SCU) 5-9