XD R stands for XD Road, and it is based off SRAM’S highly successful XD driver for mountain bikes. The simple idea behind XD and XDR drivers was to allow SRAM to bring you a wider gear range by allowing you to run a smaller cog than can currently be fitted on standard freehub bodies.
SRAM created the XDR and XD drivers to allow us to run cassettes that had cogs with less than 11 teeth, which is great if you want to go and run a 1x drive train and still have a big top gear. It will help to eliminate the topping out that some of us were getting on the flat and the much more likely topping out when we were descending.