Red Hat JBoss A-MQ Tuning Guide at
https://access.redhat.com/site/documentation/en-US/JBoss_A-MQ/6.0.EA/html/Tuning_Guide/files/GenTuning-FlowControl.html
says:

memoryUsage—for non-persistent messages, specifies the maximum amount of memory used to hold the messages.

In fact, it is incorrect since the memoryUsage specified the maximum amount of memory allocated to the broker.

Fuse ESB 7.1 documentation has the same error at:
https://access.redhat.com/site/documentation/en-US/Fuse_MQ_Enterprise/7.1/html/Tuning_Guide/files/GenTuning-FlowControl.html