Introduction:
For years we came across various fields similar to lookup fields but with data types such as customer and activity party which did look like lookup fields and also their functionality has been designed on the conceptual architecture of the lookup field. These type of fields were only present out of the box in Dynamics CRM, and no custom fields of type customer or activity party could be made. But with the commencement of first update to Dynamics CRM 2016 there is now provision to create custom customer type field.
In the previous versions of Dynamics CRM we did come across customer type of fields several times. So we are quite familiar with the architecture and behaviour of such fields for example consider the Company Name (parentcustomerid) field on the contact entity form.
Here you can chose either account or contact
Working:
- A custom customer type of field can be created in the similar manner as of other fields just go to Settings -> Customizations in the CRM then select an entity for which you want to create a custom field. While setting the data type for the field you can find the Customer data type as shown below:
- When we create a normal lookup field a single relationship is created between two entities as shown below. This relationship can be viewed in 1:N relationship of Primary entity and in N:1 relationship of Related entity. The lookup to the primary entity is created on the related entity form.
Similarly while creating a customer type of field you will see a relationship being created for Account as well as Contact entity.
- The same relationships can be viewed at the N:1 side of the Related Entity(Promotional Offer) as shown below:
And also at the 1:N side of both the Primary entities i.e. Account and Contact you find the same relationship created.
Thus by placing the customer type field on any of the entity form, one can make full use of the customer type field as per convenience. 
Conclusion:
Thus by making creation of custom customer type field possible one could see a wide scope in field customizations in near future and also we can expect other such out of the box(OOB) datatypes to be made available for creation of custom fields.
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