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

  FT_CHANNELCOMBINATION creates a cell-array with combinations of EEG/MEG
  channels for subsequent cross-spectral-density and coherence analysis
 
  You should specify channel combinations as a two-column cell array,
    cfg.channelcmb = {  'EMG' 'MLF31'
                        'EMG' 'MLF32'
                        'EMG' 'MLF33' };
  to compare EMG with these three sensors, or
    cfg.channelcmb = { 'MEG' 'MEG' };
  to make all MEG combinations, or
    cfg.channelcmb = { 'EMG' 'MEG' };
  to make all combinations between the EMG and all MEG channels.
 
  For each column, you can specify a mixture of real channel labels
  and of special strings that will be replaced by the corresponding
  channel labels. Channels that are not present in the raw datafile
  are automatically removed from the channel list.
 
  When directional connectivity measures will subsequently be computed, the
  interpretation of each channel-combination is that the direction of the
  interaction is from the first column to the second column.
 
  Note that the default behaviour is to exclude symmetric pairs and
  auto-combinations.
 
  See also FT_CHANNELSELECTION