a string which is displayed in the first line of --help output, after the usage summary programname [OPTION...]
a newly created #GOptionContext, which must be freed with g_option_context_free() after use.
ConstructionException GTK+ fails to create the object.
2.6
Creates a new option context.
The @parameter_string can serve multiple purposes. It can be used to add descriptions for "rest" arguments, which are not parsed by the #GOptionContext, typically something like "FILES" or "FILE1 FILE2...". If you are using #G_OPTION_REMAINING for collecting "rest" arguments, GLib handles this automatically by using the @arg_description of the corresponding #GOptionEntry in the usage summary.
Another usage is to give a short summary of the program functionality, like " - frob the strings", which will be displayed in the same line as the usage. For a longer description of the program functionality that should be displayed as a paragraph below the usage line, use g_option_context_set_summary().
Note that the @parameter_string is translated using the function set with g_option_context_set_translate_func(), so it should normally be passed untranslated.