bool GetIgnoreUnknownElements() const
void SetIgnoreUnknownElements(bool bValue)
 PropertyDescription 
    Property Name IgnoreUnknownElements  
    Property Type bool  
    Accessors Read/Write  
    Description Cause the parser to ignore elements that are not present in the schema.  
    Remarks

This is mutually exclusive with IgnoreMissingMandatoryElements as you can either ignore what is missing or what is extra, but not both.
E.g. expect A B C D
With IgnoreMissingMandatoryElements you can ignore a missing B, i.e. A C D
With IgnoreUnknownElements you can ignore an unknown C i.e. A C B C D
But with both set, you cannot process A C D as you don't now know if you have a missing B or if C is an unknown element without looking ahead. I.e. it could be A C D or A C B C D.