Whenever a quantum environment emerges as a classical system it behaves like a measuring apparatus

We consider a quantum principal system ?, with an environment ? made of N elementary quantum components. By using properties of environmental coherent states we demonstrate that whenever ? behaves according to an effective classical theory such theory emerges from the quantum description of a measuring apparatus in the large-N limit, regardless of the actual interaction between ? and ?. The above result implies that any dynamical map from ?? to ?? takes the form of a measure-and-prepare map whenever ? is a macroscopic system whose behaviour is effectively described by a classical theory. A connection with quantum darwinism follows.