The detection of arc lines in the wavelength calibration frame 
.bdf
is done by
SEARCH/MOS
This command will take the slitlets' limits from 
.tbl ( mos)
and the
eventual binning and stepping factor from 
( 3,3).
In all CCD rows
of 
.bdf that are
selected by these parameters it will look for intensities above the chosen
threshold when comparing to the median intensity over a chosen window
(threshold and window are defined by 
( 200,5)).
Any lines detected this
way will be centered by 
( GAUSS) and
stored in 
.tbl ( linpos).
With
PLOT/SEARCH
you will get a plot of the x-positions of all found arc lines versus the CCD rows.
If you do not know the central wavelength and mean dispersion of the grism
you used you have to identify at least two arc lines in any row of any slitlet.
IDENTIFY/MOS
The row, in which the lines shall be identified, is read from 
( 0). If you do not provide any number either there or on the command line,
the program will take the first CCD row that has been scanned.
As frame it will use 
.bdf.