I thought this at first, but having done a bit more digging, it does appear that both terms were used because they had slightly different meanings. From what I can gather, “Leave” was allocated in blocks on a rotational basis, whilst “Furlough” was issued on an individual level and usually for longer periods of time. The Leave/Furlough entries in the Routine Orders match this theory. Of course I can’t be 100% certain this is correct, but it seems to fit what I am seeing. It doesn’t affect what I am doing, I was just intrigued by it. Every day is an education!