firm-real-time

I have read the definitions for the different notions of real-time , and the examples provided for hard and soft real-time systems make sense to me. But, there is no real explanation or example of a firm real-time system. According to the link above: Firm: Infrequent deadline misses are tolerable, but may degrade the system's quality of service. The usefulness of a result is zero after its deadline. Is there a clear distinction between firm real-time vs. hard or soft real-time, and is there a good example that illustrates that distinction? In comments, Charles asked that I submit tag wikis for