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.
You should use the appengine-web.xml file for configuring your app only
if you are migrating an existing app from the App Engine Java 8 runtime to the
latest supported Java version and you want to use the legacy bundled services.
If you are using an appengine-web.xml in your project, the app.yaml is
automatically generated for you at deployment.
App Engine Java applications use a configuration file, named appengine-web.xml,
to specify information about your app and to identify which files in the app's
WAR file are static files (like images) and which are resource files used by
the application.
Syntax
An App Engine Java app must have a file named appengine-web.xml in its
WAR, in the directory WEB-INF/. This is an XML file whose root element is
<appengine-web-app>.
You can find the Document Type Definition and schema specifications for the
appengine-web.xml in the SDK's docs/ directory.