Port Arrival time is an assigned time to get in line for check-in at the port terminal.
Boarding Group is the order you enter the ship once checked in and waiting inside the terminal. They are called in order and move fairly quickly once the process begins. But early PATs may have a longer wait until the process begins.
Both have limited numbers of guests to keep the number of people waiting to a controlled amount.
Boarding Group 1 is usually reserved for those booked in Concierge Level cabins. They DO NOT get an assigned PAT. Once they arrive at the port - at their own chosen time, they have their own check-in line, often a set-apart waiting area, and possibly an escort onto the ship.
As stated above, the parking deck does have an opening time to allow departing guests to clear parking spaces.