The first number is always the one that represents the level of water resistance. This corresponds to the ability of the fabric to prevent the passage of water through the outer layer of the jacket. This measurement is expressed in millimeters. The higher the number, the higher the membrane is performing. Meanwhile, the second measure indicates the breathability of the coat, so its ability to wick moisture. This is measured in grams per square meter.
The each company offers a wide selection of models. Among these, there are different classes of membranes that offer various types of performance. So do not be shy to ask questions to the consultants in store.