Where have you read that? That would be pretty useless, it would mean that you have to stop after every parking spot, and switch to R just to see if the spot is suitable for auto parallel parking. I don't think so. And the guy
here who could already try it out clearly says the P appears on the dash as soon as you drive past a suitable spot lower than 15mph. Then, if you want to park in this spot, you stop and switch to R which activates the camera and auto parking guide on the touch screen with a button to start the auto parking process.