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

  FT_DATATYPE_SOURCE describes the FieldTrip MATLAB structure for volume data
  The vol data structure represents data on a regular volumetric 3-D grid, like
  an anatomical MRI, a functional MRI, etc. It can also represent a source
  reconstructed estimate of the activity measured with MEG. In this case the
  source reconstruction is estimated or interpolated on the regular 3-D dipole
  grid (like a box).
  An example vol structure is
            dim: [181 217 181]         the dimensionality of the 3D volume
      transform: [4x4 double]          affine transformation matrix for mapping the voxel coordinates to the head coordinate system
        anatomy: [181x217x181 double]  numeric data, in this case anatomical information
  Required fields:
    - transform, dim
  Optional fields:
    - anatomy, prob, stat, grey, white, csf, or any other field with
      dimensions that are consistent with dim
  Deprecated fields:
    - none
  Obsoleted fields:
    - none
  Revision history:
  (2011) The dimord field was deprecated and we agreed that volume
  data should be 3-dimensional and not N-dimensional with arbitary
  dimensions. In case time-frequency recolved data has to be represented
  on a 3-d grid, the source representation should be used.
  (2010) The dimord field was added by some functions, but not by all
  (2003) The initial version was defined
  See also FT_DATATYPE and FT_DATATYPE_xxx