The FITS compatability has primarily been implemented for use with the Isys interface, thus all I/O opperations within that packages can cope with either HDS or FITS file formats (with the exception that Isys will not yet read a FITS data cube). Also, the I/O can be mixed between these formats; thus a radial profile taken from an HDS can be saved in either HDS or FITS format depending upon either the output file name or any "%..." qualifier.
The XRT interface commands have also been converted to FITS RDF.
None of the HDS general commands (eg. HREAD, HDIR, &c.), with the exception of HISTORY, will yet function with FITS files.
The complex point-spread function and spectral fitting formats have not yet been converted to FITS equivalents. Though commands like PSS will work correctly given a simple FITS image.