I tried searching the forums for "etiquette", but I didn't find the answer to the following question in the threads that came up. In US SF (particularly, but I imagine it applies in other SOF too), when does one use "sir" when addressing another SF soldier?
My understanding (correct me if I'm wrong) is that it's reserved for addressing officers. But can it be used as a sign of respect between enlisted soldiers (NCOs?). Would a regular infantryman ever call an SF soldier "sir" out of respect? Or even an SF soldier to another SF soldier? What about civilians; are there different standards for them? Can it be seen as common courtesy by them?
And finally, how should a foreign SF soldier use "sir" when addressing US SF? Only towards officers (mandatory or facultative?), or can he use it for enlisted personnel too, should he wish so?
These may be obvious questions for an American, but I am of course a dirty foreigner

, so sorry!