Impossible measurements require impossible apparatus

A well-recognized open conceptual problem in relativistic quantum field theory concerns the relation between measurement and causality. Naive generalizations of rules can allow for superluminal signaling (``impossible measurements''). This raises delineating physically allowed measurements operations. We analyze this issue a recently proposed framework which local (in possibly curved spacetime) are described by coupling system to probe. show that state-update rule setting is consistent with causality provided probe local. Thus, establishing well-defined successive measurements, we also provide class Conversely, impossible only be performed using (nonlocal) apparatus.