ETP/CP and PNRs are very different.
Equi-Time Point / Critical point is a time based consideration, the point along your FPT where you are exactly x minutes to your origin and x minutes to your destination. There are many variations of this though, whether it be a normal ops , depressurization or Engine out situation.
PNR is a fuel based consideration, it is the last point on the FPT at which you can safely return to your destination.
Aircraft operate with PNRs or LPSDs generally on routes where they push their range or push there tank capacity and are unable to carry additional fuel to allow for alternates or holding due WX etc. If the forecast for arrival is below the minima by the time the a/c reaches its PNR or LPSD it is basically forced to divert or return to origin.
Quite common for A320s or 737 operating 5 hour legs to relatively remote aerodromes.