When the option to subtract the background is turned on ( NOBACKGND set to 0 in the calnicc.setup file) and the interpolation over the spectra is tuned off (NOINTERPOL option in calnicc.setup set to 1), then the background file defined in the grismspec.dat file is read, scaled, and subtracted from the data. In case the images have been chopped during observations and a background images has been generated for the grism image but not subtracted yet, the name of background image should be used.
If no background image is available, there are two option to estimate and subtract the background. The first one is to subtract pre-defined "default" background images, which are included in the NICMOSlook distribution. In the cases of the G096 and G141 grisms, this type of background subtraction often gives good results. For the G206 grism, it is currently recommended to enable the NOINTERPOL option in calnicc.setup so that the subtracted background is instead estimated by interpolating the background region surrounding the spectra of interest.