This extension provides a generic way creating a message queue. This message queue could then be used by other systems to retrieve changes from CiviCRM.
For example we can have a change message queue for submitting changes to an Outlook 365 address book. Every time a name of a contact or its e-mail address is changed. A message is added to queue. Outlook 365 can then read message by message to process the changes.
Documentation
On docs.crm-showcases.org this extension is used and explained in a show case for creating a membership portal: