Had to read the wikipedia article on Real-Time Operating System (RTOS) to figure out what it was supposed to mean. https://en.wikipedia.org/wiki/Real-time_operating_system
They compare it to timeshare systems, which seems horribly out of date (although apparently that's still kind of what occurs anyways on CPUs). What's the part that's "Real Time" relative to anything else people do with Cortex processors? The preemptive part? Not trying to be critical, just not getting the real time part. Does it not share CPU resources among tasks? Get a fixed core per task or something? Minimal interrupts and minimal thread switching?
As ab71e5 said, "Real time" in the context of OSes, means the operating system is optimized for determinism. The OS guarantees that events will be handle in a particular time-window and that high priority task will always run first.
Actually, what kind of stuff other than the stock market actually cares that much about strict determinism and task priority? Only, other that really suggests itself is schedulers for multiple groups running big iron tasks, rather than micro-controllers.
Not familiar enough, just doesn't seem like there would be a lot of groups fighting about priority on micro-controllers. Usually seem very focused and single task specific anyways.