From long time, we all know that there is a field called Customer where we can store two entities information Account and Contact.
Normally Account is used to represent a company and Contact is used to represent Individual Clients (Who are point of contact for a company). Based on need we choose Account or Contact to be stored in that field. OOB already has this type field on the following entities.
There is no way where we can create this type of field on Custom Entities till CRM 2016 Update 1. We used to create two lookup fields one for Account and the other for Contact. Now, we can create a field of type Customer to handle this
1) Navigate to Settings –> Customizations –> Select an Entity –> Fields –> New
2) Give the Display Name and select Data Type as Customer
It will create two Many to One (N :1) relation ships from the entity you have selected and 1:N relationship from Account and Contact entities.
Hope this helps