Valve rating is nothing but the maximum pressure a valve can withstand or a pressure which reducing 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)

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