What is great to see with Dynamics 365 is the concept of the Application User. This means that subscribing systems can be connected to external systems using a 'Service Account' style user rather than the previous technique of providing username and passwords.
One of the knock on effects of this is the new 'Application User' form on the User record. I've had some questions from people who see this form by default. You simply need to change the form back the standard 'User' form:
If you try to save the user record on this form you will receive the error 'You must provide a value for Application ID' since this is a mandatory field.
If you want to learn more about the Multi-tenant server to server application user integration scenarios that are used by App Source, then check out the following article: