For HIRES, this is a ThAr exposure, usually taken for 1 second with the ng3 neutral density lamp filter when using the red cross disperser.
For ESI, you should include both CuAr (about 300 seconds) and HgNe+Xe (about 4 seconds) arclamp exposures. However, only one arclamp is required.
Note that Xe can substitute for HgNe+Xe, or separate HgNe and Xe exposures could be added using opim .
In all cases, arclamps taken at around the time of the object exposure are recommended.
If the arc lamp is not given, the object will be extracted without a wavelength scale.