It "field" possession, once you reach your "place of abode" (house, condo, RV back at the camp, etc.), and process the fish, they are no longer counted towards "possession".
But they do still count towards that days limit, cant go catch another days limit just cause you filleted them back at the house.
Assuming you are fishing alone:
If you are on the pier and have more than a days limit in your cooler: ticket.
If you are headed off the pier and have more than ones days limit of fillets in the cooler: ticket.
If you are in your vehicle on the way back to your "place of abode" and get pulled over for some reason and you have more than a days limit of un-processed fish in the cooler, ticket.
Crossing state lines with fish & game is always tricky, I will ask our enforcement guys.

