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

  FT_APPENDFREQ concatenates multiple frequency or time-frequency data structures
  that have been processed separately. If the input data structures contain different
  channels, it will be concatenated along the channel direction. If the channels are
  identical in the input data structures, the data will be concatenated along the
  repetition dimension.
  Use as
   combined = ft_appendfreq(cfg, freq1, freq2, ...)
  The configuration should contain
    cfg.parameter  = string, the name of the field to concatenate
  The configuration can optionally contain
    cfg.appenddim  = string, the dimension to concatenate over (default is automatic)
    cfg.tolerance  = scalar, tolerance to determine how different the frequency and/or
                     time axes are allowed to still be considered compatible (default = 1e-5)
  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.