What is the difference between Quattro and Quattro Ultra in the Audi A4?

I have been looking at different Audi A4 models and I noticed there are some with Quattro and others with Quattro Ultra. What exactly is the difference between these two all-wheel drive systems? Are they just different names or do they offer different benefits?

Quattro Ultra is an updated version of the standard Quattro system, designed to improve fuel efficiency. It is able to disengage the rear axle when not needed, reducing drag and therefore improving gas mileage. However, this also means it is less capable off-road or in extreme weather conditions.

Both Quattro and Quattro Ultra are all-wheel drive systems, meaning power is distributed to all four wheels for better traction. The key difference is that Quattro Ultra uses an electronically controlled multi-plate clutch to engage or disengage the rear wheels, providing more flexibility and adaptability. In general, Quattro Ultra is more efficient and responsive, while Quattro is better for off-road or extreme conditions.