Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help fieldtrip2besa”.

  FIELDTRIP2BESA saves a FieldTrip data structures to a corresponding BESA file. This
  export function is based on documentation that was provided by Todor Jordanov of
  BESA.
 
  Use as
    fieldtrip2besa(filename, elec)
  to export single trial data as a set of ascii-vectorized files (.avr)
 
  Use as
    fieldtrip2besa(filename, elec)
  or
    fieldtrip2besa(filename, grad)
  to export channel positions (.elp).
 
  Additional key-value pairs can be specified according to
    channel   = cell-array, can be used to make subset and to reorder the channels
 
  See also FIELDTRIP2SPSS, FIELDTRIP2FIFF