Flat washers are great for adding resistance to your bolts and screws to prevent them from boring into the surface and weakening it.
Use a copper washer where ductility or electrical conductivity is required.
Copper resists rust in wet environments.
If there is an "(S)" in the part name, it indicates that it has a smaller O.D, compared to the "(L)" which indicates that it has a larger O.D
For the product numbers that are sold by weight, 5lbs or 40lbs the individual pieces that are contained within the package are approximate due to manufacturers making some flat washers/lock washers slightly thicker, or thinner.