Denier is a measure of fiber thickness. One denier is 9,000 meters (5.5 miles) of yarn weighing one gram.
High numbers mean thick and heavy yarns that are used in strong fabrics.
Low numbers mean finer fibers.
For packs, 500 denier to 1000 denier fabrics are common.
A 70 denier fabric is strong enough for clothing, and is often used in shells.
For down or other insulated garments, 30 denier fabric is common as the outer fabric.
Well, 1000 denier is a hefty kind of fabric for underwear, but then again, my underwear is bulletproof.