Sets the value of MullionThickness attribute. This attribute represents thickness of the mullion (vertical separator of window panels within a window), measured parallel to the window elevation plane. The mullion is part of the lining and the mullion depth is assumed to be identical to the lining depth. If the MullionThickness is set to zero (and the MullionOffset set to a positive length), then the window is divided horizontally without a physical divider.

IFC4 CHANGE: Data type changed to IfcNonNegativeLengthMeasure.