Regarding the selection of flanges

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The supplementary provisions for Class A2 fluid pipelines in GB50316 state that flat welding (plate-type) flanges should not be used. How should this be understood? Can PL flanges not be used for non-pressure pipelines either? Can a SO flat-weld flange with a neck be used? Additionally, both the chemical process design manual and the chemical piping design manual contain a table showing the appropriate types of flanges to use; this table indicates that for generally flammable, explosive, and moderately toxic process media, WN flanges are used. Which standard does this come from? Besides situations involving extreme hazards, high levels of hazard and moderate hazards due to strong permeability, as well as severe cycling and high-temperature pipelines, what other circumstances require the mandatory use of WN flanges? Which standards should fine chemical enterprises refer to when selecting pipe flanges? GB50316&HG/T20583?