When evaluating rain jackets, the most critical metric is the Waterproof Rating (measured in mm). For professional B2B sourcing, look for a minimum of 10,000mm for general outdoor use and 20,000mm+ for heavy-duty workwear. Additionally, Breathability (measured in g/m²/24hrs) is essential to prevent internal condensation; a rating of 5,000g to 10,000g is standard for active use. Ensure the construction features fully taped seams (not just critically taped) and DWR (Durable Water Repellent) coatings that are PFOA/PFOS-free to meet modern environmental standards.