(1) If a tootle-flute and a drone pipe or two are attached, what you actually have is a bagpipe (aka agony pouch) with shoulder straps.
(2) For a long distance backpacker with an especially hearty appetite, this is a shoulder-strap-equipped bean bag, which handily doubles as a camp chair. The advantage of a pack stuffed with actual beans instead of plastic pellets is that beans have more calories and hurt less on the way out. Plus they are biodegradable, as you may know from personal experience. And they make their own music too, saving you the trouble of learning to play the bagpipe and having to dodge small arms fire from your hiking companions.
(3) A hiker hump.
(4) Pack, or anyway the thing that truly makes a pack a pack. It doesn't matter if you have an external frame pack, an internal frame pack, or a frameless pack, the bag's the thing. You can't do much with a frame all by itself, or just shoulder straps and a hip belt, and anyway the bag determines what the frame is internal or external to.
Yes, you could have a pack bag made of anything really (like cedar shingles, or fiberglass, or welded titanium), but as you'd guess from the word bag, fabric is the real deal. You probably wouldn't be happy with any fabric substitute, no matter how shiny, termite-resistant, or aromatic. With a correctly-sized and fitted pack bag you can carry as many cabbages or rutabagas as you need to see you through a trip, if any, or if you frequently fall into lakes and streams for example, you can augment your air supply with a windbag attachment (which, however, requires expert seam sealing).
Then again, as mentioned earlier, just stick a couple chanters on it and presto, you have your very own agony pouch to play on those lonely nights in camp when you can't sleep because you no longer have any friends. (This scenario is an example of what is called a "vicious circle" or "rotating rathole".)
(5) The fabric sack that holds all your stuff, a bag carried on your back or shoulders by means of one or more straps.
A pack bag really is the important part of a backpack. Without it all you have is straps, and that arrangement would not only identify you as stupid but would look pretty weird as well. But you're a hiker, so what the hey.
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Me? Really this weird.
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