The basic mechanism is very similar to the check-in / check-out. Who occupies a parking slot has to check-in, which communicates that there is a busy place at a certain space coordinate.
This information becomes public when the owner announces that he is freeing the parking slot (check-out): the place appears as 'freeing' to anyone who is trying to park in the area, and can be booked and eventually complete the exchange.Looking for a parking
If you are looking for a parking you can launch the search ether by the mobile phone installed on suitable support in your car, or from the navigator, when in the area where you'd like to park. In a few seconds you see parking just freed or 'being freed' in the area, filtered depending on the cost or the fact that they are in a secured or not.