This will just be a general overview of how the office backstage works.
There have recently been a number of requests asking how the office backstage works with office 2013 or office 365. First I want to explain the office backstage is different from the "connect to office" feature. The backstage is the portion of office under file that resembles the below image, where connect to office resembles explorer view/mapped drive.
There are two types of locations in the backstage. There are the account/service locations(also called Native Services) and the locations you've manually added(sometimes referred to as Mounted Services).
Manually added locations are very straight forward as they just require you to put in the orgid/Microsoft account and password tied to them and they get added. These services get added to the registry and are saved. It's the native service locations that can cause confusion.
Native Service locations are created by the Service Manager Cache through Exchange's Autodiscover. Office uses that AutoDiscover to find the On-Premises SharePoint settings (SPMySiteHostURL) as long as you have Autodiscover setup (again usually via exchange) then the service should find any sharepoint/onedrive accounts tied to the user account currently accessing office.
In environments with both On-premise and SharePoint Online. If the account is federated then both SharePoint Online and On-Premise should show in the backstage by default. If not federated then only On-Premise will show by default, but SharePoint Online can be added manually as a Mounted service.
I hope this clears up any confusion with regards to the Office Backstage.