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

  FT_APPENDDATA combines multiple datasets that have been preprocessed separately
  into a single large dataset.
  Use as
    data = ft_appenddata(cfg, data1, data2, data3, ...)
  where the configuration can be empty.
  If the input datasets all have the same channels, the trials will be
  concatenated. This is useful for example if you have different
  experimental conditions, which, besides analyzing them separately, for
  some reason you also want to analyze together. The function will check
  for consistency in the order of the channels. If the order is inconsistent
  the channel order of the output will be according to the channel order of
  the first data structure in the input.
  If the input datasets have different channels, but the same number of
  trials, the channels will be concatenated within each trial. This is
  useful for example if the data that you want to analyze contains both
  MEG and EMG channels which require different preprocessing options.
  Occasionally, the data needs to be concatenated in the trial dimension while
  there's a slight discrepancy in the channels in the input data (e.g. missing
  channels in one of the data structures). The function will then return a data
  structure containing only the channels which are present in all inputs.
  To facilitate data-handling and distributed computing you can use
    cfg.inputfile   =  ...
    cfg.outputfile  =  ...
  If you specify one of these (or both) the input data will be read from a *.mat
  file on disk and/or the output data will be written to a *.mat file. These mat
  files should contain only a single variable, corresponding with the
  input/output structure. The data structure in the input file should be a
  cell array for this particular function.