HardenGetBackout attribute in Chapter 42 "Attributes for queues" in the WebSphere MQ Application Programming Reference.
HardenGetBackout parameter in the "Change, Copy, and Create Queue" command in the WebSphere MQ Programmable Command Formats and Administration Interface.
HardenGetBackout parameter in "Queue attributes" in Appendix C "Event data for object attributes" in Monitoring WebSphere MQ.