Whenever a code is specified. For example, when its a coded vessel ie., a vessel that is designed and built to code and must be code stamped (certified) - Such as one made to ASME I, VIII etc, \If the design requires welding in construction (or repair), then within those codes you would get directed to the welding code ASME IX.
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*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***
*** 'Pressure-relief Valve' is the equivalent ASME/API term ***