Lundberg-Palmgren theory expresses the number of stress cycles to failure as N. When applied to gas turbines, N has come to mean the rotational speed that can be applied to a component that will give statistically infinite life. N1, usually the compressor stage, is the speed that you can whizz the compressor stage round at forever, N2, the power turbine etc. You get percentages higher than 100% N1 or N2 used because you accept the statistical reduction in disk life in order to obtain the transient increased power. Other factors limit engine life more than how fast you spin the turbine round so it is an acceptable trade off.
I am attempting to dredge up knowlege from many years ago and it might be because N was Frank Whittles favourite letter. Or it might stand for noisy. Or something else. And if it is something different I'd really like to know.