My understanding is that unspecified behavior is poison as the standard understands it. The standard pretty much days that implementation being free to do whatever it thinks is reasonable. I'm not deeply familiar with the nuances of LLVM's terminology here, but that would be frozen poison I think?
Unspecified means you pick a value at a specific point in time (the standard defines this point in time) and then you treat is a regular value. It is not the same as poison. It might be similar to frozen poison, but I am not 100% sure about clang does there.
