Often in Ferret command files the identical argument substitutions must be repeated at several points in the file. Using symbols it is possible to write "cleaner" Ferret scripts in which the argument substitution occurs only once‚ to define a symbol which is used in place of the argument thereafter. See the demonstration script symbol_demo.jnl for an example.
yes? go symbol_demo airt L=3
See also "Arguments to GO files" for much more about writing GO scripts which take arguments.