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

  FT_DATATYPE_VOLUME describes the FieldTrip MATLAB structure for volumetric data.
  The volume 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 volume structure is
        anatomy: [181x217x181 double]  the numeric data, in this case anatomical information
            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
           unit: 'mm'                  geometrical units of the coordinate system
       coordsys: 'ctf'                 description of the coordinate system
  Required fields:
    - transform, dim
  Optional fields:
    - anatomy, prob, stat, grey, white, csf, or any other field with dimensions that are consistent with dim
    - size, coordsys
  Deprecated fields:
    - dimord
  Obsoleted fields:
    - none
  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 array.
  (2012b) Ensure that the anatomy-field (if present) does not contain
  infinite values.
  (2012) A placeholder 2012 version was created that ensured the axes
  of the coordinate system to be right-handed. This actually never
  has made it to the default version. An executive decision regarding
  this has not been made as far as I (JM) am aware, and probably it's
  a more principled approach to keep the handedness free, so don't mess
  with it here. However, keep this snippet of code for reference.
  (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