RMC
Speaking of experience I had on the 767, all the time I put the IRS to OFF after the shutdown check-list, I get a NO COMM message on the ACARS/DATALINK screen.
I guess the datalink, especially the satcom module, needs to know where the aircraft is sitting in order to auto-tune the right sat channel (and probably also the VHF ACARS, as the frequency changes according to the region you are located).
Never made an in-depth research on this, though.
BF