I believe that they are assuming that you have the speed log and the flux-gate compass installed already. One of my bones of contention with these companies, of particular concern in internet sales, is that it like pulling hen's teeth to get the details you need pre-purchase. Whether this stems from a history of customers purchasing primarily through a knowledgeable dealer and not direct, I do not know. But it would be hardly the first time one opened the box to find the "installation" manual only to discover facts that no one seemed to know pre-purchase.
The West Marine catalog usually has more information than the manufacturers provide within their technical articles and this case is no exception; you need velocity information, either from GPS or speed log and electronic compass.
Now as to what is compatible......
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.