Match Rule and Match validation Rule

HI All,

Please let us know the scenario when the Match validation rule will applicable.
Because as of my knowledge the match validation rule will check the below scenarios:
It will compare the invoice and shipment refnum qualifier and service provider after the shipment and invoice get matched.

But where the match rule itself do the below validation:
if the service provider and service provider alias will match between shipment and invoice then only it will allow to match them, even the validation has been done in initially at match rule then why the match validation will come in to scope.

Please clarify on this scenario

Regards
Raj