PCAL sets the mode of the pulse cal generator on the VLBA. Note that this can also be controlled from the main schedule. Most setup files will set this to the 1 MHz setting. If it should be turned off, as for spectral line observations which might be confused by the presence of the tones, it is likely that the user will want it on for some scans (calibrators, for example) and off for others. In such cases, it is most convenient to control it from the main schedule.
This currently (Oct 2001) cannot be controlled by means of the VEX file. Most MkIV stations only support ``1MHz'' or ``off''. Around the EVN, 1 MHz insertion is the default and switching it off is still a manual operation at most stations that must be brought to the attention of the local Friend of VLBI.