You probably already use validation rules, and now you can create them for custom metadata fields. Use validation rules within custom metadata type fields just as you do for non-custom metadata type fields. You can also create validation rules for relationship fields.
For example, let’s say you create a custom metadata type named Employee Records. The type contains the relationship field Feedback__c, which has an entity relationship. You can create a rule that traverses the relationship to check if the related object is a custom object or field.
The syntax is like any other validation rule, but instead of choosing a field, you select the entity relationship field that you created.
EQUALS(RIGHT(Feedback__r.QualifiedApiName, 3), '__c')
If you use relationship fields, you can use validation between two custom metadata types. You can also use them on the following targets:
|Field||Entity Definition||Field Definition|
|Qualified API Name|