Documentation

Split by Legend

This setting will be available once you have added data to the Legend field. If this toggle is enabled, the visual will split this series into subsegments that can be stacked or clustered (depending on your selected Display setting).

While the Split by Legend (Series Defaults) setting affects all series, you can use the toggle here to individually enable or disable splitting for each series, overriding the default setting.  
combo-legend-series-config