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Durable Water Repellent - also called DWR. A fabric treatment that improves wind and water resistance but allows the fabric to remain breathable. Can be a chemical wash-in or spray-on treatment to an existing fabric or can be incorporated into the material's manufacture. Generally comes in 1.1 oz and 1.9 oz weights, and useful for projects requiring breathability like hammock, windsuits, quilt shells, etc.
You could use it for a tarp, and just hope and pray that it doesn't rain for very long. Most use the waterproof silnylon for tarps...