We found out that Windup Ruleset XSD should use required element attributes. Sometimes they are defined as

<... use="optional"/>

and sometimes without that. The optional is the default so we probably need to decide which element attributes are mandatory according to java code which assume some mandatory/optional conditions.

This appears to be a similar issue with element definitions. By default, elements have a minOccurs of 1 which basically means required. If an element is truly optional, then it's minOccurs needs to be set to 0 explicitly.