Don't recall ever using terms shelving and unshelving alarms, but others I have used.
Enable/Disable: Disabling alarm means that even if the alarm conditions are met, it will not be displayed as alarm in the system. Enabling means that the alarm will be displayed again.
Suppress/unsuppress: Suppressing alarm means that it will disappear from the system, even if it is still active. After defined time it will appear again, or when it gets active again. I think the shelving and unshelving might mean the same as suppressing/unsuppressing.
Disabling/Enabling is quite risky feature and there should be at least notification that there are alarms disabled visible at all times (and somewhere a list of them).