Add Validation Rules for Custom Metadata Type Fields

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.
EQUALS(RIGHT(Feedback__r.QualifiedApiName, 3), '__c')
The syntax is like any other validation rule, but instead of choosing a field, you select the entity relationship field that you created.

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
Data Type - Checkmark.
Developer Name Checkmark. Checkmark.
Namespace Prefix Checkmark. Checkmark.
Qualified API Name Checkmark. Checkmark.