Hi Kate,
I have one but have not used it “in anger”. To produce a spectrum calibrated in absolute flux, two sets of spectra are taken, one as normal in the narrow slit, and one effectively “slitless” using the wider part.
The narrow slit spectrum has the fine detail and accurate wavelength calibration but only includes the fraction of flux which happens to pass through the slit so cannot be used to measure the absolute flux. The wide slit spectrum has lower resolution and poorer wavelength calibration but includes the total flux from the target. By combining the two a fully flux calibrated spectrum can be produced with accurate wavelength and good resolution. Christian Buil’s page here shows how to do this using ISIS.
http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm
and an example of it in use (in French)
http://www.astrosurf.com/buil/alpy600/photometric_slit.htm
The conventional photometric brightnesses can then be calculated by integrating the spectrum over the wavelength range of the standard photometric filters.
More commonly in the amateur realm, the reverse is done ie a spectrum is produced as normal using a narrow slit and then calibrated in absolute flux using brightness measurements obtained using photometric filters.
EDIT added a link