What is the difference between the Quattro and ultra Quattro systems in the Audi A6?

I am researching the Audi A6 and came across the terms Quattro and ultra Quattro. Can anyone explain how they differ and which one would be better for driving in various weather conditions?

The Quattro system in the Audi A6 is the more traditional all-wheel drive system, distributing power to all four wheels to improve traction. On the other hand, the ultra Quattro system is a more advanced version that primarily powers the front wheels and only sends power to the rear wheels when needed. The ultra Quattro system is better for driving in inclement weather as it can save fuel and provide a smoother driving experience overall.