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This defaults to Position when creating a new Bar of Pie or Pie of Pie series.

Setting this to Custom will set UseDefaultSplitValue to false. If this was custom, setting it to anything else will set UseDefaultSplitValue to true.

This property is linked for series in the same group. See SettingsPieDoughnut for more information on how linking works.
For series that are not Bar of Pie or Pie of Pie series, setting this will have no effect and it will always return Position.