One definition might be - and I'm not going to claim this is definitive - that (Maximum) Instantaneous Turn Rate is the highest rate of turn 9or, if you prefer, load factor) that can be achieved for the speed, weight, configuration, etc. under discussion but without regard for the energy state of the aircraft, whereas (maximum) Sustained Turn Rate requires an additional constraint that the aircraft be able to maintain a stable energy state i.e. the engines be capable of maintaining the speed.
I've seen that differentiation applied to military aircraft turn capability, where the instantaneous rate is likely limited only by structural considerations, certainly at higher speeds, but the sustained rate can be very limited by thrust available.