If Column to group by is left blank and the report has no groupings, the running total is calculated based on the report's current sort order, if one exists.
If Column to group by is left blank and the report has groupings, the lowest grouping level is automatically used as the Column to group by.
If a column is specified for Column to group by, the running total resets and recalculates within each distinct value of that column.
Sort order affects all running total calculations, as it determines the sequence in which values are accumulated.