Schwalbe positions the XR as the "ultimate expedition tire" with excellent puncture resistance and durability. They do not promote it as an off-road tire even though it comes in a very mountain-bike like 50-622 (as well as 26" MTB sizes).
However, to what extent can the XR's be used off road? They have a tread, but I sure wouldn't call them knobbies. I've used them on degraded chip seal, but can they be used efficiently on double track? What about deep sand and mud? For those of you with experience where would you and where would you not go off-road with the XR?