A source model (souce.model image) or components list is converted into model visibilities that is inserted into the MODEL_DATA column or alternatively is stored  in the header of the MS to be served on the fly when requested.  This is needed to use more complicated sources than setjy provides; e.g resolved source or off centered sources in gaincal.  (Setjy will automatically make this ft step.) 

The sources currently available are 3C48, 3C138, 3C147, 3C286 at 1.4, 5.0, 8.4, 15, 22, 43 GHz.  Their location is site dependent.  In Charlottesville and at the SOC, the models are in /usr/lib/casapy/data/nrao/VLA/CalModels.