Function arg_sort_by 

pub fn arg_sort_by<E>(by: E, sort_options: SortMultipleOptions) -> Expr
where
    E: AsRef<[Expr]>,

Available on crate feature lazy only.

Find the indexes that would sort these series in order of appearance.

That means that the first Series will be used to determine the ordering until duplicates are found. Once duplicates are found, the next Series will be used and so on.