Most RTOS have between 32 and 256 possible priorities for individual tasks/processes. The scheduler will run the task with the highest priority. When a running task gives up the CPU, the next highest priority task runs, and so on...
The highest priority task in the system will have the CPU until:
- it runs to completion (i.e. it voluntarily give up the CPU)
- a higher priority task is made ready, in which case the original task is pre-empted by the new (higher priority) task.
As a developer, it is your job to assign the task priorities such that your deadlines will be met.