I have a 2011 XF.

I want to be able to distinguish between private use and business use as I use it for my holiday let business.
I know there are 3 trips - auto which resets to zero every time ignition off; trip A and trip B. I know I can access and reset the trips A and B from the vehicle setting screen.

However what I dont understand is why if I specify trip A that the mileage I do on trip A adds on to the cumulative for trip B.
So If for example I have a reading of 10 miles trip A and 5 miles trip B; then if I choose trip B and do 12 miles then B will show 17 miles but if I look at trip A it will then show 22 miles.

Also I dont understand the reason why I would add on last trip to the auto trip.

So my question is - how to use the trip computer to record cumulative mileage for each trip so I can accurately distinguish between my journeys by simply choosing A or B trip at the start of the journey.

Thanks