What is "valve rating" and "Valve class" ?what differencre

Valve rating is nothing but the maximum pressure a valve can withstand or a pressure which valve designed.

Class are meant for Flange design.

The rating of a valve if you take B16-34 is the maxiumum pressure (psig) at a specified temperature (deg F) for a specified material (ASTM/ASME).

A Class covers all the ratings for the particular material within the specified temperature range.

For example Class 150 for WCB would cover all the pressure ratings from -20F to 1000F for that material. (It also covers both standard and special class)

Hope this helps.