Can you find the auxiliary heat board on the wiring diagram?
Your Daikin dealer should be able to call the factory for any direction he needs... they have generally been pretty good about that.
I see these sorts of references frequently, many times not relevant to the task at hand but a way for the draftsman or oversight engineer involved to avoid putting such directions in his operations manual where it belongs... usually because he does not know what those directions should be or the part numbers etc.
Generally speaking, maybe not in this case, I would'nt know, the sales brochures do not have any useful technical service information in them.
See if you can find the aux heat IO board in your unit, let me know what you can discover, very often there is a tiny adjustable device built into the printed circuit card with a screw driver slot in it, and a pointer that you can use to set the aux heat cut in point... if the card exists it will also have two terminals that you can wire an outside air sensor to. You may need to buy that sensor only the factory would have its specification.
I would press the Daikin factory on that point... it is not a common issue, that type of unit was only a small part of Daikins line, some of their help desk people may not even be aware of the issue.
Let me know what you find, we can go from there.