The AA tanks within convoys are capable of launching missiles well outside the range at which you get the IFF marker. So for example, let's say I do an attack run on a randomly spawned convoy. I don't destroy all the AA tanks in one run. When I turn around for a second run, the tanks are launching missile at me, but I can't return fire because the IFF lock-on square doesn't appear until I'm about than 3 miles from a vehicle, forcing me to attempt to lock on well within the range of the Inferno missiles, where I would rather be locking on from the missile's maximum range of 4 miles. However, it's impossible to fire the Inferno at its maximum range of 4 miles, because the convoy vehicles don't have IFF tags until you're within 3 miles of them.