Measurability is a stronger condition in comparison with observability. To be able to measure a system state, suitable scales must be applied (cf. Mohaghegh and Mosleh, 2009b) and some measuring device must be applied. In the case that the system only satisfies the observability criterion, a state estimator has to be ap-plied, which is collecting output information over time.
Controlling safety entails that control functions can keep the system states in desired regions of the state space and that controls can be used to prevent movements to unsafe states and/or by some means transfer the system to a safe state. A system model contains information about how to make these direct or indirect transitions from unsafe to safe states.