Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help fieldtrip2fiff”.

  FIELDTRIP2FIFF saves a FieldTrip raw data structure as a fiff-file, allowing it
  to be further analyzed by the Elekta/Neuromag software, or in the MNE suite
  software.
 
  Use as
    fieldtrip2fiff(filename, data)
  where filename is the name of the output file, and data is a raw data structure
  as obtained from FT_PREPROCESSING, or a timelock structure obtained from
  FT_TIMELOCKANALYSIS.
 
  If the data comes from preprocessing and has only one trial, then it writes the
  data into raw continuous format. If present in the data, the original header
  from neuromag is reused (also removing the non-used channels). Otherwise, the
  function tries to create a correct header, which might or might not contain the
  correct scaling and channel location. If the data contains events in the cfg
  structure, it writes the events in the MNE format (three columns) into a file
  based on "filename", ending with "-eve.fif"
 
  See also FT_DATATYPE_RAW, FT_DATATYPE_TIMELOCK