as has already been mentioned, "E" is for most of Europe. The second letter "D" for Germany from "Deutschland". Most large international airfields have a second "D", then the letter fitting it, if available.... like Frankfurt= EDDF, Hamburg=EDDH, etc. After the wall fell, all civilian fields kept the "ED__", the military ones were then given the "ET__" formerly used by the East German fields, as already mentioned.
Other fields are given indicators relating to the next big airport plus their own name, when possible. Egelsbach near Frankfurt has EDFE (FE=Frankfurt / Egelsbach), EDFM=Mannheim.
Since you can only have so many combinations with 4 letters, some indicators make no sense... though sometimes still do if you look at them closer. Aschaffenburg has EDFC. F=Frankfurt, C= VOR "Charlie", located nearby. Or a small field north of Frankfurt, EDGR=Giessen-Reiskirchen. Some may have historical meanings that are no longer clear.
But does it really matter?
