The bivouac sack / bag shortened to bivy bag is a usually a waterproof lightweight cover that goes over your your sleeping bag. A bivy bag is not for beginners as they are not tents and you do not have room to mover around ; That is if you are sleeping under the stars.They can be used as a survival bag and stashed away in your rucksack.For survivalists the bivy is the way to go for light wild camping or for those long treks where you really need to cut down on weight and test yourself.
Bivy bags covers that are the same shape as a sleeping bag ( and can be used on their own in good weather ) and give waterproof protection when without a tent. It cam be described as an other layer over the sleeping bag or an ultra thin sleeping bag.They were originally used as survival or emergency shelters and have since evolved into lightweight breathable bags and some come with lightweight poles more akin to a one man tent but much lighter and more compact than any one person tent
We get all kinds of survivalists and wild campers coming into our shop.They do not really want the comfort of a tent but want to experience the outdoors and have more freedom to roam : or travel on long distance routes, moving quickly ,such as the West Highland Way. This type of hiker is not interested in resting in a camp setting.
Almost anyone can use a bivy;-