Message
xref
has @ref-type="aff" but does not point to an <aff/> or <aff-alternatives/> element.
Rule XPath:
This rule REPORTS an alert when the following is TRUE:
@ref-type = 'aff' and not(every $rid in tokenize(@rid, '\s+') satisfies key('element-by-id',
$rid)[self::aff or self::aff-alternatives])