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

  FT_DATATYPE_HEADMODEL describes the FieldTrip MATLAB structure for a volume
  conduction model of the head that can be used for forward computations of
  the EEG potentials or the MEG fields. The volume conduction model represents
  the geometrical and the conductive properties of the head. These determine
  how the secondary (or impressed) currents flow and how these contribute to
  the model potential or field.
  A large number of forward solutions for the EEG and MEG are supported
  in FieldTrip, each with its own specification of the MATLAB structure that
  describes the volume conduction model of th ehead. It would be difficult to
  list all the possibilities here. One common feature is that the volume
  conduction model should specify its type, and that preferably it should
  specify the geometrical units in which it is expressed (e.g. mm, cm or m).
  An example of an EEG volume conduction model with 4 concentric spheres is:
  headmodel =
         r: [86 88 94 100]
         c: [0.33 1.00 0.042 0.33]
         o: [0 0 0]
      type: 'concentricspheres'
      unit: 'mm'
  An example of an MEG volume conduction model with a single sphere fitted to
  the scalp with its center 4 cm above the line connecting the ears is:
  headmodel =
         r: [12]
         o: [0 0 4]
      type: 'singlesphere'
      unit: 'cm'
  For each of the methods XXX for the volume conduction model, a corresponding
  function FT_HEADMODEL_XXX exists that contains all specific details and
  references to literature that describes the implementation.
  Required fields:
    - type
  Optional fields:
    - unit
  Deprecated fields:
    - inner_skull_surface, source_surface, skin_surface, source, skin
  Obsoleted fields:
    - <none specified>
  Revision history:
  (2015/latest) Use the field name "pos" instead of "pnt" for vertex positions.
  (2014) All numeric values are represented in double precision.
  (2013) Always use the field "cond" for conductivity.
  (2012) Use consistent names for the volume conductor type in the structure, the
  documentation and for the actual implementation, e.g. bem_openmeeg -> openmeeg,
  fem_simbio -> simbio, concentric -> concentricspheres. Deprecated the fields
  that indicate the index of the innermost and outermost surfaces.