You can be charged for a service in different ways. The details about how you're charged we call a 'plan'. Sometimes you'll have more than one plan for a service, e.g. you might have 2 users on a Basic plan at $10/month and 5 users on a Professional plan at $100/year.

The “owner” is the person in the organization responsible for a plan. 

A user can be the owner of more than one plan for each service.

If you set an alert for a service the owner is the one being notified.