Sometimes the terms we apply to garments and clothing
The term 'waterproof' used to mean that a garment was completely impervious to water. In original designs this was done through layers of treatment that prevented water from soaking completely through the fabric (even though it did absorb some). Now that the textiles industry is introducing breathable materials for professional industries, even a high-quality garment with seam sealing tape isn't necessarily completely waterproof.
There are some natural and artificial elements that are fully waterproof but they're few and far between. Clothing is not one of them, at least not for most consumers. Metal, glass and garments used in hazardous situations are. The rest of us consumers buy water resistant clothing that is proofed against water to some degree but they're not fully waterproof. Each garment touting water repellant technology is rated and measured using standard criteria.
Rain Room Endurance - Rain rooms are research labs designed to do just what they sound like. These rooms rump controlled amounts of water on a garment to simulate real life conditions. Unfortunately this doesn't replicate real life conditions where water is concerned and so it's an ineffective way of fully showing off the waterproofing of a garment.
Static Column Testing - This is the most effective method for testing the waterproofing of a garment. While it does not emulate real world conditions, this test is able to check how water transfers through a garment under pressure. A 1 inch tube is placed upright on the fabric, flush with the surface. The tube is then filled with water. The measurement is the height of the water column in millimeters when leakage starts to occur.
Learn more about Seam Seal Tape at LoxyTapes.com
The combination of these two test produces the ratings that we see on a lot of garments, especially internationally. These ratings are intended to tell us just how waterproof a particular garment is compared to others in the same category. Unfortunately the numbers can be a little confusing unless you actually know how to interpret them and what the ranges are. They go from 0 (a screen door on a battle ship) to well into the thousands (50,000mm+) for professional watertight clothing. Unfortunately these aren't real world tests, because real world conditions cannot yet be replicated consistently.
While a lot of casual consumer clothing isn't 100% waterproof, some manufactures use seam sealing tape to help reduce water leaks. The downside here is that some of those seam sealing tapes are put on using poor quality PU adhesive that wears away quickly. Once it becomes damage the water can easily collect at the seams and soak through. The best garments will have maximum waterproofing of the fabric with high quality seam sealing tape all around.