The waterproof rating of a jacket is all about how long it will take for water to begin to permeate the jacket, or its ability to keep you dry in snowy or rainy conditions. The higher the number, the longer the jacket will keep you dry when wet, which keeps you warm.
Very High Waterproofing (>20,001mm) means that are the best way to stay warm and dry. By combining the best waterproof fabrics and best water repellant DWR coating, these jackets will stay dry all day in sustained snowfall and moderate rain.
Jackets with High Waterproofing (15,001mm-20,000mm) are great choices for avid skiers and snowboarders that need a jacket to withstand any conditions they might encounter. These jackets will keep you warm and dry in heavy, wet snow and rain.
Jackets with Moderate Waterproofing (10,001mm-15,000mm) are the most common and will typically use a combination of water repellant DWR coating and a waterproof fabric. These jackets will keep you dry in light to moderate snow all day long and in light rain.
Jackets with Mild Waterproofing (5,001mm-10,000mm) can use a waterproof fabric or a more advanced water repellant coating called DWR. These will keep you dry in average snowfall and light rain.
Water Resistant (<5,000mm) jackets are treated with a water repellant coating called DWR. These will keep you dry in light snow but will start to absorb water quickly in the rain
Not Specified means that the manufacturer's catalog claims that the product is Water Resistant or Waterproof, but doesn't provide an exact measurement.
Not Treated means just that. These jackets are not treated with any additional waterproofing and are typically spring/fall jackets or insulator jackets that are meant to go underneath a more waterproof jacket.