As I understand it, from a basic point of view:
1. the closer you are to the ground (whether on departure or arrival) the lower the RNP value required.
2. the further you are from an airport (and land) the higher numerical value of RNP required. In other words, Oceanic sectors (with no radio position updating) maybe ok with RNP10.
3.Carrying out an approach to the FAF or even lower, will require an RNP of better (lower) than 1, probably 0.3 or better.
Hope that helps