The RESPFILE option is the one which deals with spatial respones created by SPRESP. If the default offered at the PSF prompt is 'RESPFILE' this means that the dataset has an attached spatial response. Accepting this default means that the psf system will use the information in the response - supplying an alternative ignores the response.
If the RESPFILE option is selected for a dataset which has no energy axis, but the response does have a defined energy axis, the the AUX parameter is used to select an energy value. This situation might arise where you have attached a response to a spectral image, but have then PROJECTed this down to a 2D image to do some (for example) image processing.