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.