It was technically classified Confidential (only certain parts) when I was using it regularly in the late 70s & early 80s. It was generally declassified in the early 2000s.
There is a VERY long list of two letter (flag) combinations. Bravo & Zulu just happened to be the next combination when the creators got down to the message "Well Done".
I have a Bravo Zulu sticker on my car to go with my "Spy Navy" bumper sticker. 
WWG1WGA
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)