The bottom extension plane ID option. 
   Namespace:   Autodesk.Revit.DB.Structure  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 22.0.0.0 (22.1.0.0) 
  Since:  2015 
Syntax
Remarks
 This Plane identifies a Level, a Grid, or a Ref Plane. To validate plane use  [!:Autodesk::Revit::DB::Structure::AnalyticalModelColumn::isPlaneValidForExtension]  method. 
 Exceptions
| Exception | Condition | 
|---|---|
| Autodesk.Revit.Exceptions ArgumentNullException | When setting this property: A non-optional argument was null |