sql sum function parameters on teradataml dataframe

Question

I have a teradataml dataframe, and I have the following code :

tdml_df = tdml_df.assign(stock=func.sum(tdml_df.flow.expression)
                                   .over(partition_by=tdml_df.NUM_CTR.expression,
                                         order_by=tdml_df.month_out.expression.asc(),
                                         rows=(None, -1)))

Do you know what rows=(None, -1) is for ? Thank you !