How does Office Online determine the datacenter to route a given user to?

When using Office Online to view and edit documents stored in external (non-Microsoft) storage, users are typically routed to the Office Online datacenter closest to their current geographic location. However, this routing is not guaranteed, and users may be routed to any global datacenter for a variety of reasons.

Thus, Office Online does not guarantee that data will stay within a particular geographical region when used to view and edit documents stored in external (non-Microsoft) storage.