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There are five occurrences of FOAY0001 in XP31 but it is not linked (to FO31, where it seems to be defined) or defined in that spec.
Actually, I see that all FO errors in FP are not linked, so maybe this is by design. But I find it confusing, as it is not clear in the text that it is about an FO error and searching for it in the spec does not give you the definition. So I propose, if at all possible/feasible, to link these external errors or to give them a generic "see FO31" marker or something.