Rigid pavements because of their nature, allow for a larger variation in subgrade material (in terms of strength), unlike flexible pavements. That is why rigid pavements can be built over almost any kind of soils with appropriate ground improvement.
The general range for CBR is 2% to 14% or 15%. If it is found below 6%, then the code specifies some necessary subgrade improvement before laying the concrete slab directly over it.
Also, the CBR could be increased up to say 40% or 50% by ground improvement, and still, the subgrade may be fit for rigid pavements.