You don't find the variable anywhere.
It's a variable you ( as a Airplane Gauge developer ) can write to, to instruct GSX not deboard pilots or crews, so it's used by airplane developers, which might already have their own internal simulation of the number of crew/pilots or the fact they should board or not, so we left it as a developers-only variable, in order not to add another possible layer of confusion in users seeing conflicting reports from GSX and the airplane.
