This dialog allows selecting a substitute user for a user that is established as out of the office. You can access it by setting the Availability option to "User is currently out of the office", this option is available in the Account or Users menu.
When a substitute user is configured all the tasks in the user's inbox are redirected to the substitute's inbox. In addition, when the original user returns, tasks are no longer redirected and those tasks not processed by the substitute user are removed from his/her inbox and kept in the original user's inbox.