Seat pressure rating is the maximum pressure the seat is capable of repeatedly and reliably operating at. It normally goes down as temperature increases, so manufacturers publish pressure-temperature ratings. Often, but not always the case, the seat has a lower pressure rating than the valve body. Especially when the valve has plastic or rubber seals.
There is no industry stanadrd for establishing pressure-temperature ratings and it is left up to the manufacturers to determine their product's limits. It is normally done by testing, but some of the low cost manufacturers just copy the data from their competitors catalogs. Each Manufacturer has different amounts of liability they are willing to extend in order to hype their products, so you will see different ratings from different manufacturers even for the same materials. this is a great question to ask a valve supplier. If they can explain how they do it, then in general, you can trust the product will perform to the ratings they advertise.
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