Complex sentence: Unlike Regression Tests, to which they are very close, characterizion tests do not verify the "correct" behaviour of the code, which can be impossible to determine, but rather to verify the behaviour that was observed when they were written.
Split into simpler sentences: Unlike Regression Tests, to which they are very close, characterization tests do not verify the "correct" behaviour of the code, which can be impossible to determine. Instead they verify the behaviour that was observed when they were written.