Parking is pretty simple as driving, all the player has to do is to stop the vehicle at his/her desired location and exit. Over the years of GTA parking has become slightly more realistic than just simply parking and exiting the car.
Parking in 3D Universe was just simply stop and exit, if the player would park in the street, the headlights would still be on and the engine would idle as if it had been left untouched. The only time vehicles would shut off if they were parked in garages and saved, that was the only way to perfectly park. Another possibility is to download and install a modification that allows the player to turn the engine off. Other than that, it was just stop and exit.
In the HD Universe parking has become realistic unlike the earlier GTA Eras where it was stop and go, now it has become complicated. Players can still stop and go and leave the engine running, but now players can turn off their vehicles by holding the exit button without releasing it until the character exits. This will enable the car to turn off and shutting down the radio, engine, lights, etc. leaving this ability more realistic. If the player turns the vehicle off, they will have to start it on again as if it had been in that place. The vehicle will also shut off if left idle for a long period of time.