Slot 0 will be where it is saved.

 

Plug it in, save your map, download a map, load the new map , disconnect the FP3 and ride the bike. Your ECM will now have the map you downloaded and use it.
To go back to stock, plug it in, select stock map, flash, unplug and your ECM will now be back like it was before the FP3.
You can leave it plugged in and under the side cover, use your phone to access it to make adjustments or view data as needed. or unplug it and put it in your toolbox when done.

From what it sounds like what you want to do, the first sentence will suit you just fine, and you will always have your stock map stored.