CombinationMethod on form.
[Visual Basic .NET] Public Property CombinationMethod As esriSelectionResultEnum
[C#] public esriSelectionResultEnum CombinationMethod {get; set;}
Product Availability
Available with ArcGIS Desktop.
Remarks
The combination method defines how the selection performed will combine with an existing selection. It is the programmatic equivalent of the pulldown menus found in the selection dialogs in ArcMap.