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Note that there is no explicit prioritization scheme reflected in this process, but that implied prioritization does take place. The fact that tree transversal proceeds from the root member toward leaf members gives members closer to the root a higher priority. Hence, the hierarchical structure of the IST determines the system's fixed interrupt priority structure. Conversely, a transversal ISR is free to use any algorithm to decide which child member's ISR should be invoked--for example, an anti-starvation algorithm or a priority based on the value of InterruptMemberNumber. Whatever method is used, transversal ISRs provide the dynamic aspect of system's interrupt priority structure. Implementing the IST structure and ISR usage sets the implied prioritization of all interrupts.
The Mac OS does set priority for hardware interrupts, separately from the software interrupt dispatch mechanism. It is therefore possible for hardware interrupt priority to cause an ISR to be interrupts.