What is the important aspect of a real-time system or Mission Critical Systems?

A real time operating system has well defined fixed time constraints. Process must be done within the defined constraints or the system will fail.

An example is the operating system for a flight control computer or an advanced jet airplane. Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.

Real-Time systems may be either hard or soft real-time.

Hard real-time: Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM), Conflicts with time-sharing systems, not supported by general-purpose operating systems.

Soft real-time: Limited utility in industrial control of robotics, Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.