The three letter and/or digit codes are the package code you are booked under. Most people pay attention when there's a FD code or % off the room code or other promotion, but ALL packages have some sort of code attached to them in the Disney computer system, even the basic rack rate packages. It can vary by the resort, or season, or optional inclusions, even the booking source (i.e. booked with a TA or by Disney directly).
For example: code XYZ (totally made up for this example!) may mean: moderate resort, fall season, park hopper, basic DDP. Code XZZ (made up code!) might be the same package with the QSDP.
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