Since the standard must be suitable for different microcontrollers, the specification of interrupt handling routines only cover the general approach that a compliant OS should follow, without coping with any hardware related issues.
In particular, the standard provides two kind of ISR handlers:
The ISR does not use an operating system service. In practice, the OS does not handle these interrupts, and the designer is free to write his handler, with the only restriction that he can not call any OS service. Typically, these are the fastest highest priority interrupts.
The ISR is handled by the system, so OS calls can be called from the handler.
Inside any ISR no rescheduling will take place. Rescheduling takes place on termination of the ISR category 2 if a preemptable task has been interrupted and if no other interrupt is active. At the end of the ISR category 1 no rescheduling takes place too, and this is the reason because ISR category 1 should have the highest priority in a correct design.