You can find some more info about the use of these "tactical codes" on this site (in german) :
http://www.luftarchiv.de/
On the menu on the left choose "Luftfahrtindustrie" , then choose "1939-1945" right below "Motorflugzeuge".
So in the case of Christian's picture :
D = Name of the Commander of the new or temporary formed unit
4 = 4th Staffel
J = aircraft letter within the 4th Staffel
Hope this might help,
Rudi.