If a check table exists on a field, and we are using Automap, the natural solution is to use XREF as that would be the normal case for most fields. Even in the cases where the source and target values are the same, the methodology can be consistent in case something changes or a new source arrives.
In the case of fields with multipart keys, automap should map to RuleXref. And, where possible, the actual SQL should be provided via an override table. For most fields that have multipart keys, the logic would always be the same.