For US facilities, this classification (code, standard, RP, etc) makes no difference. That's because OSHA regards API 521 as RAGAGEP for relief design, regardless of what it's called. Therefore, and you're subject to citation if you deviate from API-521 in the US.
I got that straight from the horse's mouth (OSHA). This came up during a discussion of this topic. We were discussing if there's any significance, from an enforcement perspective, between an API document being called an RP or a standard. The OSHA enforcement leader from Washington DC responded saying it doesn't matter what you call it. They view API 520 and 521 as established RAGAGEP, which means compliance is necessary. Those who have been through an OSHA NEP audit knows this already, because OSHA's citations are full of specific references to API521 and 520.