It's maybe late to give an answer but check this amazing article on Medium.

I like to think of the Application Registration as an abstract class, and the Service Principle as the class that inhirts it. You have one GLOBAL application registration, for all your tenants, and one or more LOCAL service principle objects, one for each tenant. When we first create an ApplicationRegistration, a local service principle is created automatically for the current Tenant (home tenant).

An Application Registration object has:

• 1 <--> 1 Relashionship with the software application (your phisical application).
• 1 <--> n Relashionship with the corresponding Service Principle objects.