If the Captain says that he is going on his 4th mission in this rotation, then he's a flight commander.
Otherwise, nobody uses but I think its in some paperwork or books.
I guess if you run into someone that says that, you could ask them about what password the Inflight Commander is using.
Hopefully in 20 years we'll start calling rotations by their more common name, trips. 2 day trip, 3 day trip, 4 day trip, 12 day trip.