Message
@ref-type is required for an xref that points to string-join(key('element-by-id', tokenize(@rid, '\s+'))/local-name(), ', ') elements.
Rule XPath:
This rule ASSERTS the following, raising an alert if FALSE:
@ref-type or not(key('element-by-id', tokenize(@rid, '\s+'))[self::aff or self::aff-alternatives
or self::corresp or self::bio or self::table-wrap or self::table-wrap-group or self::fig
or self::fig-group or self::fn or self::ref or self::aff])