End Point for the Integration Engine for
Acknowledgments 
An end point of this type is defined for a sender system that expects an acknowledgment from the receiver of its message.
To define this type of end point, proceed as described in End Point for an Integration Engine, but instead of entering type=receiver in the URL, specify type=sender_back.
Also specify the following values:
For the receiver interface name: ACKNOWLEDGMENT
For the receiver interface namespace: http://sap.com/xi/internal

You can only create acknowledgment end points for an Integration Engine for SAP systems acting as receiver systems.
Also see:
For more information, see the Processing of Acknowledgment Messages section in this documentation.