The different Codes / Standards used for design of Pressure Reducing Valves.
Please narrow the question.  Thousands of standards exist.  International and US standards differ.
The AWWA standards for the water works industry are different than the API standards for oil and gas industry such as API Spec 6D and Std 598.  A recent post includes the MSS standards.  ASME publishes many applicable standards.  Start with B16.34 and B16.10.
Perhaps you could also clarify which pressure reducing valves you are specifically addressing.  Examples could be self-contained pressure regulators, control valves, pressure safety relief valves or other valves that control pressure downstream.  ISA publishes control valve standards.  PIP publishes valve standards.  UL 842 is a standard for vlammable fluid valves.
ASTM publishes many material standards and a few related to valves.

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