Psf models are high level manipulations of the low level instrument psfs supported by the psf system. There are spatial models and energy models, and the two can be used together. The purpose of spatial modelling is to impose granularity of psf access to speed things up. An application like PSS which having determined using the hints system that a psf varies across the field, is most easily written to simply evaluate the psf at every position of interest. For some psfs this would be extremely slow, so spatial modelling by the psf system allows grids of psfs to be set up, the number of evaluations being reduced to the number of psfs in the grid.
Energy modelling serves a slightly different purpose. When processing
image data from an instrument with an energy dependent psf it is not
strictly correct to use a mean photon energy, even if the source spectrum
is monochromatic. The psf system CSPEC psf option allows a channel spectrum
of a source to be supplied which the high level data routine
psf_2d_data uses to construct a composite psf by performing a
weighted average of psfs at different energies. This is the correct way
to get the best source flux estimate for a source in 2-D image.