If a valve is supposed to be say Class 800,  what does it mean?  I am sure it is related to pressure and temperature but do not know the exact meaning.
Also if I have a valve with known capability to handle a known pressure and temperature,  how do I know what Class it is?
It's listed in the API 602. Per grade of materials are maximum allowable design pressures presented at different temperature levels.
Class 800 is applicable for Socket weld or Threaded end valves.
API Spec 6D 2002 refers to ASME B16.34 for rating tables by material groups.  No table 2.1 exists. 
API STD 602 indicates that "Class 800 is not a listed class designation, but is an intermediate class number widely used for socketwelding and threaded end compact valves."  Also "This International Standard is applicable to valve end flanges in accordance with ASME B 16.5 and valve body ends having tapered pipe threads to ISO 7-1 or ASME B 1.20.1. It is applicable to extended body construction in sizes and pressure designations of Class 800 and Class 1500, and to bellows and bellows assembly construction as may be adaptable to gate or globe valves in sizes. It
covers bellows stem seal type testing requirements."

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