bool GetIgnoreMissingMandatoryElements() const
void SetIgnoreMissingMandatoryElements(bool bValue)
 PropertyDescription 
    Property Name IgnoreMissingMandatoryElements  
    Property Type bool  
    Accessors Read/Write  
    Description Causes the parser to ignore mandatory elements that are missing in the XML document.  
    Remarks

This is mutually exclusive with IgnoreUnknownElements 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.