Actually, it's "Army/Navy"
Goes back to WW II (no Air Force) when aircraft were being built by so many diffferent companies that a better standard was needed to reduce maintenence overhead. It's not just about flexible to ridgid, it's all kinds of lines, adapters, etc.
I use them for just about everything any more as the fittings and braided lines have become relatively inexpensive. Back when all this stuff was military specification ("MILSPEC") it was outrageously expensive - I have a couple of leftover AN-18 45 degree fittings for my airplane that were like $80
each. But now there are alternate brands to Aeroquip like Russel and others that aren't aircraft-ready but are just fine for automotive use...and so they're way cheaper.
I just offered it as a thought; running long rubber lines can be problematic and using cheap hard lines (not stainless) can end up costing less and being a far-better long-term solution even if it does cost a bit more