FLUX applies a standard star flux curve to a spectrum, replacing raw counts/sec/Angstrom (output from ALIGN) with fluxes in F-Lambda units (ergs/sec/cm/Angstrom). The flux calibration is taken from a standard star flux curve computed using the FLUXSTAR command.
FLUX will work on spectra that do not have exactly the same wavelength scale as the flux curve by fitting a spline to the flux curve, and then applies that spline fit to the spectrum.
If your spectrum has exactly the same wavelength scale as the flux curve, either because the flux standards were taken with the same spectroscopic setup as the star being fluxed, or because the spectra have been made to have the same scale with ALIGN, then you may flux calibrate the object spectrum simply by multiplying the spectrum by the flux curve with MULTIPLY command.