Let me simplify this further. One you sleep in (bivy bag), one you sit in (bothy bag), and one you 💩 in (biffy bag).A bivy bag (bivouac sack) is basically a water resistant sleeping bag cover. Some are more elaborate with a pole, headroom, and mosquito netting to be used like a single person tent. A bothy bag is also known as a mountain hut. It’s a bag that you get inside and sit on the sides (has no floor other than the piece you sit on) and use your bodies for poles (it may also allow use of trekking poles in center for added supports). They start at 2 person and go up to 12+ person sizes. I carry a 4-6 person Rab shelter which weighs 22 ounces. I’ve found ways to use it for a single person shelter in bad weather. I got that size because it is large enough for one person to lay down in. If you’re in a situation where you need one, most likely someone is injured or exhausted and needs to lay down and sleep. 4-6 person is also a good size for a small group or family hiking. And don’t confuse either a bivy bag or a bothy bag with a biffy bag. The owner will not appreciate it.