What is the internal protection provided by the silicon such as discharge diodes?

What is the internal protection provided by the silicon such as discharge diodes on each pin for PIC18F4620?

I/O electrostatic discharge (ESD) protection on this device is a simple diode stack for each pin tied to VDD and VSS. Any pin voltage ~0.7 V above VDD or ~0.7 V below VSS activates the protection.

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