Each manufacturer has its own way to make tents waterproof, UV resistant and breathable at the same time. The waterproof rating of the material is in millimeters, which is the specification you are looking for when comparing tents. The number is between 800 mm and 10,000 mm.
The number refers to the height of the water column (in millimeters). For example, 1500 mm means that the fabric will withstand a 1500 mm (5') water column for more than one minute, and then a single drop of water may appear on the fabric. This intensity is sufficient to prevent rainwater from leaking in a 75-mph hurricane. In the outdoor industry, they use this number as a shorthand to describe the amount of paint used to make nylon or polyester tents waterproof. Some people may call this value the hydrostatic head pressure.
The general consensus is that starting from 800-1000mm, this material is waterproof. However, it is best to use a high water column because the coating will wear out and the water resistance value will decrease.
The larger the value, the longer the waterproofing ability of the material, and the higher the durability of the tent; but the weight of the material (hence the tent) will be significantly higher. Long-distance cyclists usually choose tents with high Waterproof Resin Coating ratings.
PU (polyurethane) coatings are one of the strongest and most wear-resistant coatings, but manufacturers usually don't tell them which coating to use, so you need to contact them to find out. PU optimizes water resistance and strength while maintaining light weight. The manufacturer can only maximize the amount of paint at a certain point. More coating means more weight and more cost. The chemicals in the coating also weaken the fibers, so at some point, excessively increasing the water column value will reduce the strength of the fabric. This is why 10000 is the highest value you can find.