The IG receptacle has a grounding connection separate from and redundant with the EGC that will bond the enclosure (if metal) and the yoke of the receptacle. The theory is that the sensitive electronic device plugged into the receptacle will be compromised by the stray voltages and currents present in the normal EGC. So, some engineers decided to invent this redundant IG receptacle. The IG is run all the way back to the service ground, and kept insulated all the way.
Some of us feel that this is dumb.
I recall years ago when this issue came up, the engineers wanted to disconnect the ground altogether, and for us to drive a ground rod in the middle of the building (and sometimes bond to the building steel), and use this for our grounding conductor. They would spec that the last few feet/inches of the EMT run would be changed to PVC, and then the sensitive device would be bonded to the ground rod. They called it a static charge drain.
It worked, but the NEC folks determined that this ground would not facilitate the operation of the OC device, and the installation was unsafe. The IG is the compromise they worked out.
