What is difference between ANSI b 16.5 class 150 and ANSI b 16.5 class 1500?

The Class ranges of ANSI Standards refers to the pressure and temperature limits of usage for the Product. The class ranges from 150 and goes upto 2500. They denote the different pressure ranges.

For example, A 316 Stainless Steel valve in class 150 can be used only upto 275psi

Same 316 Stainless Steel valve in class 1500 can be used only upto 3600psi

Does that clears your doubt?