With Sram's introduction of their new Red Etap AXS group, they also announced their new XDR freehub body. XDR isn't actually new though: the first place we remember seeing it was on Sram's 900 series of hubs, which came out back in 2016. When asked about the new driver body, which a the time had no apparent use, we were told that it would allow for the development of a 12 speed road drivetrain at some point, and now that has arrived!
So what's the actual difference between an XD and an XDR driver? The latter has splines that are 1.85mm longer, making the driver body that carries the cassette effectively longer (the same as an 11-speed road Shimano-style driver, but more on that in a moment). If you want to run an 11 or 12 speed mountain cassette, you just need a 1.85mm spacer behind it on the XDR driver, and you're good to go. It's worth emphasizing, however, that while the freehub is very similar, it is longer, so it needs a hubshell that's designed for it. This is one reason that it Chris King is offering it only on their road hubs, which are built for a longer 11-speed road Shimano style freehub body to begin with. Since White Industries makes all of their hubshells slightly narrower to fit an 11-speed road freehub, the XDR freehub will work on all of them.
Because all hub brands have approached freehub and hub design in general in their own way, it's hard to make any blanket statements about compatibility across multiple brands, but here's what we can say about the brands we carry:
Beyond this basic run-down, we're sure there will be questions, and we'll be here to answer them!