The value defines the spacing between each baluster instance inserted in the excess length. 
Namespace: Autodesk.Revit.DB.Architecture
Assembly: RevitAPI (in RevitAPI.dll) Version: 24.0.0.0 (24.0.0.0)
Since:  2019 
Syntax
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Remarks
 The spacing is used only if a baluster family is set to excess length fill. 
Exceptions
| Exception | Condition | 
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| Autodesk.Revit.ExceptionsArgumentOutOfRangeException | When setting this property: The given value for excessLengthFillSpacing must be between 0 and 30000 feet. |