Spectral-line observing with VLBI is significantly different to the standard ALMA case. First and foremost, the VLBI data are only taken in a single spectral window at a fixed sky frequency of 86.268 GHz, this being used to observe the SiO line with a rest frequency of 86.243370 GHz. None of the parameters of this spectral window can be edited by the user. The other three basebands can be used to define additional spectral windows that will produce ALMA-only data. Their frequencies can be freely chosen, so long as a valid tuning solution can be found.
Another aspect of spectral-line VLBI that is non-standard is that the user must enter an observing date for the observation using the time-constraint interface (single visit). This is needed in order for the OT to confirm that the full width of the line will fall into the central 234 MHz of spectral window on the expected date, given the source's velocity relative to the telescopes. For this reason it is also necessary to define the expected line width in Expected Source Properties. If the line cannot be observed then there will be a validation error.