Hey there!
As many noticed there is now a system which generates a custom registration when the user does not enter one. While this system currently is overpowered it also does not respect the rules of specific countries.
For example while N746QA would be fine for most non military US aircraft disregarding the aircraft type, size or weight D-QGSL would not be a valid registration for a rather light GA aircraft like the C172 in Germany.
In Germany the first letter after D- is categorized by MTOW, type or amount of engines:
D-AXX > 20 t (FA7X, DH8D, A320, A380…)
D-BXX 14 – 20 t (DH8A, J328…)
D-CXX > 5,7 – 14 t (SF34, CJ4, JU52…)
D-EXX = single engine, < 2 t (C172, PA28, DR400…)
D-FXX = single engine, 2 – 5,7 t (PC12, AN2, C208…)
D-GXX = multi engine, < 2 t (DA42…)
D-HXX = helicopters
D-IXX = multi engine, 2 – 5,7 t (DA62, P180, CJ1, PA42…)
D-KXX = motorized gliders
D-LXX = airships/zeppelins/blimps
D-MXX < 600 kg ultralight aircraft (VL3, FK9…)
D-NXX = hang gliders
D-OXX = manned balloons
D-PXX = unmanned aircraft
D-1234 = gliders without engines (four random numbers after D-)
While this could be simplified it would increase realism already if specific rules per aircraft category would apply according to the general national letter (so an aircraft with letter D- which is a category “helicopter” aircraft should have a default registration starting with D-H, so “D-HAS”, an aircraft from category “airliner” should start with D-A, so “D-AXS”).