What is the Garmin 6-pin Y-cable

Hello
I am nearing the end of a boat rebuild project and fitting tthrugh hull transducers. I have tried to go for an all n2k network and thought I was on the right track with a garmin gmi 10 and sensors and buy an MFD closer to launch date. I was after a speed sensor and thought because I had already fitted a b60 transducer there was no need for the dst800 so bought the cheaper st850 with garmin part number 010-10365-00. Foolishly I fitted the collar and then only while checking it out noticed that there was a 6-pin Y-cable. I contacted the garmin support via email asking what it was and could it be re-wired and if it was compatible with the gps 5xx range how did it connect. The response was "not compatible with gmi 10 buy a dst800". Now the collar is already fitted so I can not take it back to the shop. It looks like the dst800 has a different collar so I would rather buy an adapter, re-wire it or buy another sensor to slot into existing collar. I see there is a wiring diagram on the airmar website if I used a field install able adapter would it work on n2k? The problem is I do not even know what protocol this sensor uses the Airmar website just says there is a analogue and n2k version but makes know reference to any product numbers. I Have no idea even what you use a 6-pin Y cable for or what it is supposed to connect to