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

  FT_DATATYPE_SOURCE describes the FieldTrip MATLAB structure for data that is
  represented at the source level. This is typically obtained with a beamformer of
  minimum-norm source reconstruction using FT_SOURCEANALYSIS.
  An example of a source structure obtained after performing DICS (a frequency
  domain beamformer scanning method) is shown here
            pos: [6732x3 double]       positions at which the source activity could have been estimated
         inside: [6732x1 logical]      boolean vector that indicates at which positions the source activity was estimated
            dim: [xdim ydim zdim]      if the positions can be described as a 3D regular grid, this contains the
                                        dimensionality of the 3D volume
      cumtapcnt: [120x1 double]        information about the number of tapers per original trial
           time: 0.100                 the latency at which the activity is estimated (in seconds)
           freq: 30                    the frequency at which the activity is estimated (in Hz)
            pow: [6732x120 double]     the estimated power at each source position
      powdimord: 'pos_rpt'             defines how the numeric data has to be interpreted,
                                        in this case 6732 dipole positions x 120 repetitions (i.e. trials)
            cfg: [1x1 struct]          the configuration used by the function that generated this data structure
  Required fields:
    - pos
  Optional fields:
    - time, freq, pow, coh, eta, mom, ori, cumtapcnt, dim, transform, inside, cfg, dimord, other fields with a dimord
  Deprecated fields:
    - method, outside
  Obsoleted fields:
    - xgrid, ygrid, zgrid, transform, latency, frequency
  Revision history:
  (2014) The subfields in the avg and trial fields are now present in the
  main structure, e.g. source.avg.pow is now source.pow. Furthermore, the
  inside is always represented as logical vector.
  (2011) The source representation should always be irregular, i.e. not
  a 3-D volume, contain a "pos" field and not contain a "transform".
  (2010) The source structure should contain a general "dimord" or specific
  dimords for each of the fields. The source reconstruction in the avg and
  trial substructures has been moved to the toplevel.
  (2007) The xgrid/ygrid/zgrid fields have been removed, because they are
  (2003) The initial version was defined