The survival bag is just a sheet of orange plastic approx 1.8M by 0.9M which can be used in an emergency as a sleeping bag. Unlike gore-tex they are not breathable and you end up both soaking wet and an attraction to all the creapy crawlies, also if you have your head in it you can lack oxygen leaving you with a splitting headache - assuming your lucky enough to wake up at all
In the military they have an orange bag and a green bag the idea is using both you would place an insulating layer of say leaf litter in between them. In a survival situation you would use the green as the inner and the orange as outer, in combat you would use the orange as the inner and the green as the outer
Really only used in an emergency, that said having a piece of orange plastic sheet measuring 1.8M by 1.8M is also very useful for a number of other functions, to make a waterproof bivvy, solar still, transportion bag, panel marker, emergency poncho, rucksack liner etc....the list goes on, well worth carrying due to their small size and weight