New Data Type “Customer” has been introduced in CRM Online 2016 Update and can create in Custom Entity as well.
Customer Data Type is a combination of both Contact and Account Entities, one can choose either Contact or Account record.
We can give proper Relationship Names (To Account & To Contact) before creating this field (if required or we can use as it is).
After Creating this field two Relationships will be created one with Account and one with Contact.
Create this field and place it on the Form, Save and Publish.
Open New Record and Test this field to select either Account or Contact.