France, WWI, La Zone des Armées

[Graham Mark]

A query has come to me about censorship of civil mails in France in 1918.  I concerns an encircled five-pointed red star.  Bourguignat's "Le Controle Postal et Telegraphique Francais pendant la Premier Guerre Mondiale" tells me that a directive of Feb 1918 stated that letters from le Zone des Armees, passed without opening, had to be marked with this circled star.  He illustrated an example from Department Aube, some 90 miles SE of Paris, addressed to Paris in June 1918.
Can anyone tell me, in general terms, how far back from the front lines the military control stretched?  Did it fluctuate during the war?
Graham Mark