In the previous chapter we have seen that the construction of interacting perturbative quantum field theories is given by perturbative S-matrix schemes (def. 15.3), equivalently by time-ordered products (def. 15.31) Feynman amplitudes (prop. 15.51) , which are uniquely fixed away from coinciding interaction points by the given local interaction (prop. 15.42), but which involve further choices of interactions whenever interaction vertices coincide. This choice is called the choice of (“re”-)normalization (def. 15.46) in perturbative QFT.