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Returns the element id of the level that defines the bottom of the underlay range.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 25.0.0.0 (25.0.0.0)

Return Value
ElementIdIf InvalidElementId is returned, then the underlay base level is not set and no elements will be displayed as underlay.
