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

  FT_APPLY_MONTAGE changes the montage of an electrode or gradiometer array. A
  montage can be used for EEG rereferencing, MEG synthetic gradients, MEG
  planar gradients or unmixing using ICA. This function applies the montage
  to the input EEG or MEG sensor array, which can subsequently be used for
  forward computation and source reconstruction of the data.
  Use as
    [sens]    = ft_apply_montage(sens,     montage,  ...)
    [data]    = ft_apply_montage(data,     montage,  ...)
    [freq]    = ft_apply_montage(freq,     montage,  ...)
    [montage] = ft_apply_montage(montage1, montage2, ...)
  A montage is specified as a structure with the fields
    montage.tra      = MxN matrix
    montage.labelold = Nx1 cell-array
    montage.labelnew = Mx1 cell-array
  As an example, a bipolar montage could look like this
    bipolar.labelold  = {'1',   '2',   '3',   '4'}
    bipolar.labelnew  = {'1-2', '2-3', '3-4'}
    bipolar.tra       = [
      +1 -1  0  0
       0 +1 -1  0
       0  0 +1 -1
  The montage can optionally also specify the channel type and unit of the input
  and output data with
    montage.chantypeold = Nx1 cell-array
    montage.chantypenew = Mx1 cell-array
    montage.chanunitold = Nx1 cell-array
    montage.chanunitnew = Mx1 cell-array
  Additional options should be specified in key-value pairs and can be
    'keepunused'    string, 'yes' or 'no' (default = 'no')
    'inverse'       string, 'yes' or 'no' (default = 'no')
    'balancename'   string, name of the montage (default = '')
    'feedback'      string, see FT_PROGRESS (default = 'text')
    'warning'       boolean, whether to show warnings (default = true)
    'showcallinfo'  string, 'yes' or 'no' (default = 'no')
  If the first input is a montage, then the second input montage will be
  applied to the first. In effect, the output montage will first do
  montage1, then montage2.