"arbitrary concrete behavior" means that at this point anything can happen on the real machine. This implies that everything before this point has to behave according to the specification. "is impossible" is stronger, as the whole program could behave erratically. But having partial correctness is important in a lot of scenarios and this is why we want to have it and in "UB" it is the former and not "impossible".
In the ISO C standard, we use "unspecified" for a non-deterministic choice among clearly specified alternatives. So this is well understood.
In the ISO C standard, we use "unspecified" for a non-deterministic choice among clearly specified alternatives. So this is well understood.