Snowboard shells are designed to block wind, breathe and provide waterproofing. They don't have insulation and are a lightweight outer layer designed to work in the 3-layer system. Since you already wear a base layer and insulation layer, the shell merely provides wind and water protection.
Typically, a "snowboard jacket" contains insulation as well as the wind and waterproof exterior. Insulation may be removable, making the jacket adjustable to different weather conditions. Various materials may be used for insulation including down, fleece and other manufactured alternatives. However, not all companies refer specifically to a shell and jacket in this way, and something called a jacket may actually be a shell. Make certain that you know whether you're purchasing a jacket with insulation or simply a waterproof shell. Try the item on when possible for best results.
Many jackets are labeled with the layering construction and notated like 2L, 2.5L and 3L. These terms refer to the construction of the jacket and how the shell material is crafted. A 2-layer jacket uses a membrane that is laminated to the waterproof exterior shell; a separate liner, often mesh, protects the membrane. A 2.5-layer jacket uses a textured lining on top of the membrane, which is not an entire separate layer. A 3-layer jacket includes a waterproof membrane sandwiched between the outer shell and inner layer, all laminated together.
Snowboard jackets are also categorized by certain numbers. Waterproofing and windproofing is often conveyed in millimeters such as 10,000 mm. This number refers to the amount of water that the fabric can withstand in one day before soaking through. Generally, snowboard jackets range from 5,000 mm to 30,000 mm, with higher numbers corresponding to better waterproofing. While some jackets reach ratings as high as 45,000 mm, most jackets range between 10,000 mm and 20,000 mm.
Many jackets also include a breathability rating, which indicates how well the jacket will allow sweat and heat to escape. The technical notation for breathability usually measures the amount of water that can penetrate out of one square meter of the fabric in a day. This is measured in grams. Jackets that have a breathability rating will show a number like 5,000 g. The higher the number, the better the breathability.