Transferflow will only read 2 floats at a time that I know of.
The Main Tank, MN, is the vehicle tank that the transferflow can pump fuel into. The AX is the 98 gal auxiliary tank from transferflow.
How it handles 2 vehicle tanks I am not sure unless the vehicle combines them somehow. It could, I just haven't seen it myself. I guess that AUX is always the transferflow tank. MN, Main could be the vehicle main tank that the fuel gets pumped into from the Transferflow tank and likely the biggest OEM tank.
How does the vehicle normally handle it's 2 OEM tanks? Selector switch?
I guess if you switch to the 15 Gal OEM tank that the readings on the Transferflow computer will not change.
Manuals are worthless. Transferflow will provide in depth documentation is you contact them and ask for it. No, it isn't on their website. Ignorance prevents phone calls is their thinking. However they are very helpful on the phone.
This said I can explain how it works with a 34 gal main tank and a 50 gal aux tank. The following is left out of the manual or what is there is worthless.
First and this was plain stupid IMO on their part but required for Fords. Ford diesels have a programmed miss when the fuel level gets low to protect the injection system. ('Hey dummy you are about to run out of fuel' feeling as the engine cuts out. Stupid Ford design is to ruin an expensive injection system if you run out/low on fuel.) Ford reads this our from the dash gauge info. So the 2008 tank setup I got works differentially to in regards to the OEM dash fuel gauge. (Lets say 'old school'.) The fuel gauge would read the main tank for 15 seconds and then read total fuel. So you would watch 1/3 tank reading at ignition on and then 15 seconds later the gauge would drop and the low fuel light would come on because "total fuel" level was displayed on the gauge. 1/3 of 34 gal and then it reads total of 84 gal that is lower than 1/3 as the aux tank is empty when that main is at 1/3.
After 2008, when exactly I do not know, they started reading the main tank only on the fuel gauge. For Fords that would enter the miss part of the program with plenty of fuel and customer confusion. They decided that the fuel guage reading the main tank only was less confusing to customers. All re-programmed computers have this set now - ask me how I know...
The above two items can be changed in the setup menu - and you need their guide to do so. It takes 2 changes to make the above switch back to 'Old School' readings. That is the good news.
You don't really use the fuel gauge anymore. Rather you use the number of gal left and keep 7 in reserve. Because at 7 gal the accuracy is in question of 0 to 7 gal left. This with known economy, MPG, helps you plan fuel stops. Like 18 unloaded and 8 towing MPG's. I prefer the gauge to read total fuel left as the main tank is a moving target. (Esp because their display is hard to read while driving.)
Now for the secret level.
Key off press both buttons. Turn key on with buttons held. Release when you see letters on the display.
I usually use the flowtest to pump anything left in the aux tank into the main tank past the 0 gal reading when I am pushing it before walking to get to the cheap fuel station.
Other settings are there that you should not mess with. At least not without the documentation to do so.
So far I love the system and the gal readout helps me with the fuel planning more than an inaccurate 1/2 tank reading...
The system tends to average the readings of gallons left. So get some fuel slosh going on and the readings will become lower. Read the gal before you turn off the key after travel. The readings will be higher when you turn the truck back on and then quickly drop back down as you travel due to fuel slosh. This is why we always left 7 in the tank.