When you are installing an Access Port on your car, one of the first things that it does is to "Marry" itself to your ECU. This is what allows it to communicate with your car, and what will prevent it from communicating with someone else's car.
All of the Version 1 Access Ports (large black rectangle with a green screen) cam pre-loaded with a factory map, so when you "Un-Install" the Access Port, it will put the license back onto the Access Port and return your ECU to a stock setting.
The Version 2 Access Ports work a little differently. They don't come pre-loaded with a factory map, instead when it is "Marrying" itself to your ECU, it will actually copy the factory map, and save it in its memory so if and when you re-set your ECU to stock, you will put back the exact map that you had before you re-flashed your ECU. This is handy because if your ECU had any updates flashed on it from a dealer, you would still have those when you return your ECU to stock.
Something else important to note is that the only way to return your ECU to stock is to "Un-Marry" your Access Port from your car. There are some stock maps out there that you can load onto your car, so it will be have as stock, but the difference is that your Access Port license is still on your ECU. If you just load up a stock map and your ECU is re-flashed, it will most likely over-write your Access Port license making your Access Port unable to communicate with your car until a license is put back onto it (The Access Port would have to be sent to Cobb to do this).
Also, if you just load up a stock map, your Access Port will not be able to communicate with another car because your license is still married to your ECU.