"HD" has nothing to do with the transducer shipped with the unit or anything else.
It is two letters of the alphabet that denotes a change sometimes.
When the 800 and 900 series was updated from CCFL backlighing to LED backlighting, it got the HD in the model name. Nothing else changed.
Example 898c SI Combo to 898c HD SI Combo
When the 700 series units got updated to have the Ethernet connection, it got the HD in the model name. Nothing else changed.
Example 798ci SI Combo to 798ci HD SI Combo