One of the most abused concepts I have come across over the years. Many times, I have especially seen it to this day with Point of Sale systems, they require an isolated ground. When you ask the right questions they conceive this to be a orange receptacle with a ground wire bonded to the box and run back to the branch panel without any other grounds tied to it except at the ground bus of the branch panel. A real isolated ground can be looked at as another grounded conductor just like the neutral that is bonded at the same point as the neutral, but intended to provide the ground path for equipment designated. It is important to note that when providing a real isolated ground, the boxes conduit and metal parts leading to the equipment must be grounded with a normal grounding conductor. ( I know conduit can serve). This often means running the isolated ground through panels all the way back to where the service is. An alternate is to provide an isolation transformer to create a separately derived system.
Worse, to add to the confusion, there are some who want a bond to their metal enclosures that is directly connected to earth, and they want you to use this as the NEC required ground. Where I have seen this mostly is audio contractors, who have this concept of shielding and radio interference. This group is dead (pun intended) wrong and you must refuse to do this.
The first is often unnecessary, but if it makes them feel good fine. I have seen engineers spec out isolated ground receptacles and almost always fail to indicate the required equipment and extra wiring required in the distribution system to create the isolated ground. Over the years I have come to the concept that the main real purpose for isolated ground was, sensitive electronic equipment uses various voltages, 3v, 5v etc. to ground to operate, these voltages need to be pure, and transient grounds voltages can affect them causing data transfer errors, parity errors etc. Most computing equipment the general and commercial public uses today is not sensitive to these transient voltages and the extra work isn't necessary, but if they want to pay for it...............