Which match criteria is only available as custom in order to perform an exact match...

Party Identification Id isthe only match criteria that is only available as custom in order to perform an exact match in Identity Resolution. Party Identification Id is a unique identifier for each Party Identification record in the data model. It is not one of the standard matchcriteria that Data Cloud provides out of the box, so it has to be created as a custom match criteria. Custom match criteria allow you to use any field from the data model as a match rule. References: Create Custom Match Criteria