In the standard row-by-row option, the dispersion coefficients are kept in the table ( coerbr.tbl). The rebinning to constant step in wavelength is accomplished, row by row, with the command REBIN/LONG.
If the 2-D option described above for the solution of the dispersion relation
is followed ( TWODOPT=YES), the polynomial coefficients
in both directions are
stored in the keywords KEYLONGD and COEFYD, respectively.
This information is then used by the command RECTIFY/LONG
to resample the image in the 
space.
Data resampling can be avoided by the command APPLY/DISPERSION which generates a table with the columns :WAVE and :FLUX, each row corresponding to the central wavelength and flux of a CCD detector pixel.