BatchInboxFetcher allows you to fetch notifications that have been sent to a
user (or installation, more on that later) in their raw form, allowing you
to display them in a list, for example.
This is also useful to display messages to users who disabled notifications.
Source of a notification. This is "what" triggered the push to be sent to the
user. Push Campaign, Transactional notification, ...
Unknown means that your SDK is too old to understand a new source that has
been introduced after its release.
