FOCUS is used in VLBA test observations to specify an increment to the focus value used for the subreflector position. This is added to the nominal position known by the on-line system for the particular receiver.
Note that the schedule requires focus values in mm while tsm reports focus achieved in cm.
This parameter might be used in astronomical observations if one wished to have the most optimum focus for a given frequency for a receiver that has a significant focus variation with frequency. The new (2012) 6cm receiver (4-8 GHz) has such a variation.