hehe... I just means that everyone is flying witht their altimeters set to the same datum. That the altimeter is not always (never) showing the true altitude is accounted for by setting a transition level where pilots will transition from flying on QNE (1013.25) to the airfield QNH/QFE and therefore ensure proper altitude readouts. One ofcourse needs to take the temperature deviation from ISA into account, but its normally neglected if its not alot colder than ISA.
So when flying with 1013.25 set you are flying on a Flightlevel and when on the local QNH you are flying on an Altitude.
Got it? Im crap in explaining.
cheers /duece