Folks,
hvogt's definition of PNR is a new one to me, and I have been around the aviation business for something over 50 years, including the Sydney "college of Knowledge" courses in my early days.
PNR is the last point at which you can divert to a suitable alternate, whether it is en-route, off- route or even the departure point with "normal" reserves.. Beyond PNR you are committed to the destination, with whatever reserves are required at the destination.
Radius of action is a bit different, depending on the criteria. Radius of action to a moving base used to give me the rar-tars, what application it had to civil use was a mystery to me, as it was only applicable to returning to an aircraft carrier.
ETP is equi-time point is just that, same time out as back, unrelated to endurance. However,you can use the same basic formula to work out an equi-fuel (burn) point, which is not quite the same as PNR.