The very first step after correcting bias, dark, and overscan is to find the
edges of the slitlets. This is done by the command LOCATE/MOS.
This command takes the central trace of a flat-field and detects the edges
of the slitlet by simple threshold comparison. The values are stored in the
table 
.tbl.
The offsets in dispersion direction between the slitlets will be read from the header of the flat-field frame for FORS data and stored in :xoffset. For other data you will have to determine the offsets yourself using the command OFFSET/MOS on a wavelength calibration frame. This command will derive the offsets of all slitlets relative to the first one via correlation methods and store them in the correct column. In contrast to FORS data you will then NOT have the offsets from the center of the CCD but relative to the position of the first slitlet!
As spectroscopic flat-fields normally exhibit the spectral characteristic
of the lamp that was used to produce them you have to take out this
characteristic in order to correct the CCD senisitivity variation and keep
the original flux distribution. This is done with the command NORM/MOS.
It takes an averaged flat frame and the slit limits stored in the
table 
.tbl.
It then averages separately for each slitlet the rows, fits a polynomial
of chosen degree to the flux distribution obtained this way and divides
each slitlet by its polynomial. The actual flat-correction is then
performed with COMPUTE/IMAGE.