Explanation
Currently shipping DRd Dimming Engines have a switch which changes the fan speeds based on whether a recessed kit is installed. Early DRd Dimming Engines had a reserved header (position J2) next to the fan connection (J1). When the recessed kit was introduced for the DRd, position J2's header was replaced with a switch for the recessed option. See pictures below for reference:
Original:
New: