@Dejan_Pangercic in our view, when speaking about hard-real time systems, missing a deadline implies a system failure (which often can lead to a catastrophic consequence, though not always necessarily). We discuss this topic at https://arxiv.org/pdf/1809.02595.pdf.
Summarized, our view is the following:
Real-time systems can be classified depending on how critical to meet the corresponding timing constraints. For hard real-time systems, missing a deadline is considered a system failure. Examples of real-time systems are anti-lock brakes or aircraft control systems. On the other hand, firm real-time systems are more relaxed. An information or computation delivered after a missing a deadline is considered invalid, but it does not necessarily lead to system failure. In this case, missing deadlines could degrade the performance of the system. In other words, the system can tolerate a certain amount of missed deadlines before failing. Examples of firm real-time systems include most professional and industrial robot control systems such as the control loops of collaborative robot arms, aerial robot autopilots or most mobile robots, including self-driving vehicles. Finally, in the case of soft real-time, missed deadlines -even if delivered late- remain useful. This implies that soft real-time systems do not necessarily fail due to missed deadlines, instead, they produce a degradation in the usefulness of the real-time task in execution. Examples of soft-real time systems are telepresence robots of any kind (audio, video, etc.).
Beyond this, there’s also a nice description of the differences between hard, firm and soft at Introduction to Real-time Systems
Hard real-time software systems have a set of strict deadlines, and missing a deadline is considered a system failure. Examples of hard real-time systems: airplane sensor and autopilot systems, spacecrafts and planetary rovers.
Soft real-time systems try to reach deadlines but do not fail if a deadline is missed. However, they may degrade their quality of service in such an event to improve responsiveness. Examples of soft real-time systems: audio and video delivery software for entertainment (lag is undesirable but not catastrophic).
Firm real-time systems treat information delivered/computations made after a deadline as invalid. Like soft real-time systems, they do not fail after a missed deadline, and they may degrade QoS if a deadline is missed (1). Examples of firm real-time systems: financial forecast systems, robotic assembly lines (2).