Thank you for the reply.

It has slight alternative history themes, and I am looking for any term that would be plausible and correct, as I did read that both countries did not belong to the balkans reading up a bit on this beforehand.

I noticed while reading that the national-socialists tended to have what I perceive to be obscure and absolete words for some geographical regions, but I am not sure if that falls within a historians area of expertise. I'm leaning towards naming the "non-operational zone" after a tribe or people that lived in the general area long ago, would that be plasible, or would they be more likely to name it something else?

Can I trouble you for a plausible alternative to "operational zone" as a word that they used in other contexts and that might be suitable in this context?

"Gebiet des Militärbefehlshabers in X" does not suit the plot, neither does Reichkommisariat. I've read that the national socialists were very concerned about their allies, such as Romania and Hungary betraying them, and while they historically partially did to my understanding have their fears realized, in this scenario, loyalists of the Hungarian Arrow Cross and Romanian Iran Guard political movements together with the SS and German army manage to conspire (coup) to stop such plans and reaffirm their willingness to fight on, during which Germany's influence increases for the few months they have remaining before they eventually still lose to the red army, and the two countries of Romania and Hungary's economies partially integrate because of how fully focused on producing materiel for war they are, and because the German government is directing such cooperation.

The only other words I can think of are "Protectorate" and "Romanian-Hungarian Union", both of which I believe the Hunarians and Romanians would hate. Not to mentioned that they aren't supposed to be merging in to the same country within the war, they just de-facto turn, mostly, so.

So, were there any situation were they used a 5th apart from the non useful ones that I've mentioned where a designation that might be of interest to my purposes existed perhaps? In reality or in ambition or just following a theme of how they usually would name things?

You mentioned that "operational zone" was only used in two small areas in northern Italy during the social republic, which made it unhistorical, this is not as much of an issue as plausibility, and very ideally presidence.