Curtain wall grid lines are outside wall. If you are trying to shorten the wall, please remove grid lines, including mullions on top of them.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.2.0.0 (26.2.0.0)
Syntax
C#
public static FailureDefinitionId GridOutsideWall { get; }