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

  FT_DATATYPE_PARCELLATION describes the FieldTrip MATLAB structure for parcellated
  cortex-based data and atlases. A parcellation can either be indexed or probabilistic
  (see below).
 
  A parcellation describes the tissue types for each of the surface elements.
  Parcellations are often, but not always labeled. A parcellatoin can be used to
  estimate the activity from MEG data in a known region of interest. A surface-based
  atlas is basically a very detailled parcellation with an anatomical label for each
  vertex.
 
  An example of a surface based Brodmann parcellation looks like this
 
               pos: [8192x3]         positions of the vertices forming the cortical sheet
               tri: [16382x3]        triangles of the cortical sheet
          coordsys: 'ctf'            the (head) coordinate system in which the vertex positions are expressed
              unit: 'mm'             the units in which the coordinate system is expressed
          brodmann: [8192x1 uint8]   values from 1 to N, the value 0 means unknown
     brodmannlabel: {Nx1 cell}
 
  An alternative representation of this parcellation is
 
               pos: [8192x3]           positions of the vertices forming the cortical sheet
               tri: [16382x3]          triangles of the cortical sheet
          coordsys: 'ctf'              the (head) coordinate system in which the vertex positions are expressed
              unit: 'mm'               the units in which the coordinate system is expressed
   Brodmann_Area_1: [8192x1 logical]   binary map representing the voxels belonging to the specific area
   Brodmann_Area_2: [8192x1 logical]   binary map representing the voxels belonging to the specific area
   Brodmann_Area_3: [8192x1 logical]   binary map representing the voxels belonging to the specific area
   ...
 
  The examples above demonstrate that a parcellation can be indexed, i.e. consisting of
  subsequent integer numbers (1, 2, ...) or probabilistic, consisting of real numbers
  ranging from 0 to 1 that represent probabilities between 0% and 100%. An extreme case
  is one where the probability is either 0 or 1, in which case the probability can be
  represented as a binary or logical array.
 
  The only difference to the source data structure is that the parcellation structure
  contains the additional fields xxx and xxxlabel. See FT_DATATYPE_SOURCE for further
  details.
 
  Required fields:
    - pos
 
  Optional fields:
    - tri, coordsys, unit
 
  Deprecated fields:
    - none
 
  Obsoleted fields:
    - none
 
  Revision history:
  (2012/latest) The initial version was defined in accordance with the representation of
  a voxel-based segmentation.
 
  See also FT_DATATYPE, FT_DATATYPE_SOURCE, FT_DATATYPE_SEGMENTATION