"AK" on the yellow luggage tags means that the luggage will go onto the truck that serves at Animal Kingdom area resorts, such as the All Stars.
"DD" on the yellow luggage tags means that the luggage will go onto the truck that serves at Downtown Disney area resorts, such as OKW and SSR.
There are other two-letter codes for other areas.
The two-letter codes on the Mickey head shape provide quick visual identification when the luggage is sorted at the airport. The barcodes are scanned for tracking.
The two-letter codes do not have any direct relationship to how motorcoaches are routed.