ro.nfc.port
defines the transport used by the NFC subsystem to talk to the NFC chip. In your case the NFC chip is connected to the I²C bus with /dev/pn544 as the driver.
The libnfc also supports connecting the NFC chip to a serial port. This is done for example in the tuna platform (Galaxy Nexus I think). In this case you'll see:
setprop ro.nfc.port "COM4"
chmod 0600 /dev/ttyO3
in your init.rc
Unless you are building a platform by yourself you should not mess around with these settings. They are for system builders and they (hopefully) know what they're doing. Users won't see any benefit from modifying them.