At least with the MGM system, you have to scan your slot card each visit when you enter at which time the camera scans you license plate. I assume if you take a ticket, it matches that ticket to your plate so after you have paid at a kiosk you can exit when the camera scans you plate at exit gate.
At MGM I know from experience that if you have a new slot club card printed at casino it may take up to 48 hours for the new card to work at the gates and old cards are deactivated. I assume this is so a player can't obtain multiple cards and pass them out to friends or sell them. I did get 2 cards printed last time and both work for parking. Don't know what's the limit on active cards.
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What happens if you are in a different car. Do you have to start over with scanning of your card?