When does the balloon-change or control-error event occur

Hi developers, I have questions about balloon-change or control-error event: 1. What's the meaning of these events 2. When do the events occur? The comments of their callbacks don't mention that( https://gitlab.com/libvirt/libvirt/-/blob/master/include/libvirt/libvirt-dom... https://gitlab.com/libvirt/libvirt/-/blob/master/include/libvirt/libvirt-dom... )