ICAO mandated that airplanes have registration letters (numbers) near the tail and on the upper side of the RH wing and lower side of LH wing... there is a "minimum size" for markings as well... in USA apparently they dont observe the rule anymore... foreign aircraft still seem to abide by the rule -
Comes from Admiralty Law traditions i.e. flag showing on the tail as ships were doing in the old days...
Also useful for plane-to-plane identification. Some countries would send up fighters to spot-check airliners coming through their airspace, but for obvious reasons don't want to park just off the wing so close as to read the small reg near the tail.
The only thing you should feel when shooting a terrorist: Recoil.