I think you mean CUP . . .which is "cupperous pressure." SAAMI runs a test barrel with a hole in the wall over the chamber. They put a copper plug of a specific length in the hole and fire the load.
Then they measure the compression of the plug and that provides a "number." Don't ask me how they figure the number or what the number means. I just know that 44 magnum runs 40,000 CUP and .458 Win. Mag. runs 50,300 CUP.