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

[Han Han]

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-domain.h#L4130
https://gitlab.com/libvirt/libvirt/-/blob/master/include/libvirt/libvirt-domain.h#L3736
)
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