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

  FT_SOURCEPARCELLATE combines the source-reconstruction parameters over the parcels, for
  example by averaging all the values in the anatomically or functionally labeled parcel.
  Use as
     output = ft_sourceparcellate(cfg, source, parcellation)
  where the input source is a 2D surface-based or 3-D voxel-based source grid that was for
  example obtained from FT_SOURCEANALYSIS or FT_COMPUTE_LEADFIELD. The input parcellation is
  described in detail in FT_DATATYPE_PARCELLATION (2-D) or FT_DATATYPE_SEGMENTATION (3-D) and
  can be obtained from FT_READ_ATLAS or from a custom parcellation/segmentation for your
  individual subject. The output is a channel-based representation with the combined (e.g.
  averaged) representation of the source parameters per parcel.
  The configuration "cfg" is a structure that can contain the following fields
    cfg.method       = string, method to combine the values, see below (default = 'mean')
    cfg.parcellation = string, fieldname that contains the desired parcellation
    cfg.parameter    = cell-array with strings, fields that should be parcellated (default = 'all')
  The values within a parcel or parcel-combination can be combined with different methods:
    'mean'      compute the mean
    'median'    compute the median (unsupported for fields that are represented in a cell-array)
    'eig'       compute the largest eigenvector
    'min'       take the minimal value
    'max'       take the maximal value
    'maxabs'    take the signed maxabs value