Documentation lacks examples for 6.3
https://access.redhat.com/documentation/en-US/Red_Hat_JBoss_BPM_Suite/6.3/html-single/Development_Guide/index.html#Retrieving_Process_And_Task_Information

Instead if just describes what it can be used for, but it doesn't show how to use it.

I have found some specific examples on QueryFilter here:
https://github.com/droolsjbpm/jbpm/blob/6.3.x/jbpm-services/jbpm-kie-services/src/main/java/org/jbpm/kie/services/impl/RuntimeDataServiceImpl.java

CTRL+F "QueryFilter"