NGX, NGY, NGZ controls the number of grid-points in the FFT-mesh along the directions of the three lattice-vectors. X corresponds to the first, Y to the second and Z to the third lattice-vector (X,Y and Z are not connected with cartesian coordinates, don't be fooled by the historical naming conventions).
NGXF, NGYF, NGZF controls the number of grid-points for a second, finer FFT-mesh. On this mesh the localized augmentation charges are represented if ultrasoft (US) Vanderbilt potentials or the PAW method are used. In addition, local potentials (exchange-correlation, Hartree-potential and ionic potentials) are also calculated on this second finer FFT-mesh if (and only if) US-pseudopotentials are used.
Mind: There is no need to set NGXF to a value larger than NGX, if you do not use US-pseudopotential or the PAW method. In this case ,neither the charge density nor the local potentials are set on the fine mesh. The only result is a considerable waste of storage. In this case set NGXF, NGYF, NGZF simply to 1.
In VASP.4.X all parameters are determined during runtime, either defaults are used - Sec. 6.11 or 5.23 - or NGX etc. are read from the INCAR file, see Sec. 6.3).