There are two things, one is the definition of a probability and the other is unitarity. Unitarity is observable, that is closely connected to the information

There are two things, one is the definition of a probability and the other is unitarity. Unitarity is observable, that is closely connected to the information content of the system, and if memory serves to the evolution of the normalization factor of the wave function. So if unitarity is 1, then the interpretation of quantum field theories as probability distributions is straight forward. If it is not 1, then the theories look strange.