The Actions Inbox is not part of the official documents. It is an useful tool to enhance communications between the users in your company. Usign this functionality you can assign a task to be done by other members within your company. The task at the same time can has several action items which represent an individual task that can be assigned to the same or to different people.
The action can be associated to a document like Correspondence, Request for Information, etc.
Note: Actions are only visible to the assignee and assigned.
