Region ID
The REGION_ID is an abbreviated code that Google assigns
based on the region you select when you create your app. The code does not
correspond to a country or province, even though some region IDs may appear
similar to commonly used country and province codes. For apps created after
February 2020, REGION_ID.r is included in
App Engine URLs. For existing apps created before this date, the
region ID is optional in the URL.
Learn more about region IDs.