What does the ATS Anti-Tamper Switch function do on a key switch?

There is a magnet on the cover plate of the key switch and there is a reed switch on the key switch body itself. When the cover plate is on the key switch, the ATS switch is in one state and if the cover plate is removed from the key switch, the ATS switch will change state alerting someone that the key switch has been tampered with.  This is different then the anti pull out tab, the anti pull out tab is to help prevent the cylinder from being removed from the key switch.  There is no alert with the anti pull out tab so the ATS when the cover is removed is the part that will give someone an alert about the cover plate being removed. 

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