Title says it all. What is the difference between relative and absolute deadline. I mean the deadline is relative to what?
Given a periodic task set with deadline different from periods, and with all offsets equal to 0 (∀i, ri,0 = 0): • The best assignment is the Deadline Monotonic assignment • Shorter relative deadline → higher priority
The question is more related to the meaning of the words relative/absolute than the RTOSs perse. Relative deadline makes reference to the maximum time to complete the job without jeopardize the execution of the code, that is, from the triggering event until the end of the task. On the other hand, absolute deadline is the moment in time in which the job must be completed. So, absolute deadline is the relative deadline plus the time in which the task starts running.
For the completeness sake, relative deadline are quite useful to organize the tasks, and there is a method based on it (most probably your quote comes from there) called Deadline monotonic algorithm, the shorter deadline, the higher priority. Obviously, it is easier and clearer to work with relative deadlines than absolute deadlines, being the last one useful for methods such as Earliest Deadline First.