Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_source2grid”.

  FT_SOURCE2GRID removes the fields from a source structure that are
  not necessary to reuse the dipole grid in another source estimation.
 
  Use as
    [grid] = ft_source2grid(source);
 
  The resulting grid can be used in the configuration of another
  run of FT_SOURCEANALYSIS.
 
  See also FTSOURCE2SPARSE, FT_SOURCE2FULL