While creating a custom field in dynamics 365, for data type drop-down we see one option as Customer,although Customer is NOT any entity in Dynamics 365. What the heck is that? chillax, I am going to explain this here.
What is this for?
Basically, this will create a lookup field for Account and Contact both. It follows the fact that the Client can be a Company or an Individual.
So if you are coming from the programming background, this is the broadly the Concept of Polymorphism as we are using one thing for multiple purposes and if you are not then it is killing two birds with one shot.
More about this …
When you start creating a new field for an entity having its datatype as Customer, you notice two relationships first to Account and then to Contact. So basically it creates two one to many relationships with your entity to account and contact.
On the form, it gives you search result from both of the entities, as in below image.
Moreover, if you choose to Look up More Records, you will see this and you have the option to select the required entity, as shown below.
Hope this gives you a fair idea of Customer data type.