Typically used when categorizing certain fittings such as flanges.  Classes 150 and 300 are common.  They are noted and referred to as 150# or 150LB.  This does not mean that they are rated for 150psi, as pressures vary according to temperature of the fluid/gas.  Class 150 is a standard of flange thickness and bolt pattern and size.  There are classes from 150 to 2500.