With many military surplus items, you also might need a crash course in etymology.

Coming from the German Swiss term Beiwacht (bei by, and wacht watch or patrol) the proper term for this item is is biv·ou·ac (/ˈbivo͞oˌak/) but most know it as the Bivy. When inclement weather catches you off guard, or your quick stay in the woods doesn't require a tent, the Bivy fills in the gap as a quick-to-deploy rudimentary shelter. 

Part of the Modular Sleep System (MSS), the bivy cover is a way to turn your sleeping bag into a quick shelter that will keep you warm & dry without a tent. Providing a barrier against wind chill or rain, it's an extra 5-10 degrees of insulation you'll be thankful for. Unlike the rest of the system, it's also covered in a proper camouflage to enable troops to have a low-vis sleep in enemy territory where a tent might give them away. 

Simply slide your MSS Patrol Bag, Intermediate bag, or other sleeping bag inside (MSS bags can be physically attached to the bivy for convenience) and you have a mostly weatherproof cover in a pinch.

What makes these bivy covers special is that they are vapor permeable. In layman's terms, old waterproofing fabrics trapped humidity and moisture inside as well as keeping them out, so modern materials like Gore-Tex actually have breathability to allow for less condensation. This is crucial for avoiding Hypoxia, or lack of oxygen inside of a sealed shelter, and is a game-changer for wet & snowy climates where a Bivy may be necessary. 

Specs

• Made in the USA by Tennier Industries
• Compatible with the US Modular Sleep System Patrol, ICW, and ECW bags
• Machine Washable (see instructions below or on tag)
• Vapor-Permeable, M81 Woodland Gore-Tex™ construction
• Full length zipper down the side
• Cinchable hood

Condition

Excellent: Bivy's exhibit minor to no stains or blems, and absolutely no tears, holes or rips. May exhibit writing.

Good: Bivy's exhibit clear cosmetic blems, usually in the form of an ink stain or grass stain. Bivy will still have absolutely no tears, holes or rips. May exhibit writing.

Laundering Instructions

This bivy bag and the other components of the Modular Sleep system are machine washable, each with different instructions. For the bivy bag:

wash in a standard commercial washing machine and dry in standard commercial dryer set on it's low tumble setting. Wash and rinse at less than 100 degrees Fahrenheit using low cycle. Dry at less than 100 degrees Fahrenheit using low tumble dry cycle.