Resolving power generally refers to the spatial frequency where MTF goes to zero, or the shortest distance between two objects which are just distinguishable in the image.
Since a 1200x1800 image resampled from FX has a Nyquist frequency of 25lp/mm, it can only contain detail at frequencies up to that limit. Nikon's MTF for this lens is nonzero for 30lp/mm therefore its resolving power is higher than 30lp/mm. I would guess it is quite a bit higher than that in most areas of the frame.
