Spies are a special rank of angels that tend to work alone or collaborating with military platoons outside Heaven, most often in Hell. While spies are an important asset for Heaven's military, they are not a part of the military as such.
Angels working as spies have to give up their halo (and thereby their wings) to go unnoticed in Hell or wherever else they are needed. The lack of a halo prevents them from entering Heaven on their own, meaning they usually live a life rather secluded from other angels. Being a spy is in many ways an unpleasant occupation, and not many choose this path on their own. Traditionally, the position has been given as an alternative to becoming a fallen angel, or as a middle ground between already being a fallen angel and being let back into Heaven.