When linked or imported geometry is merged together (such as by layer with the weld option) or, when 2D objects become attached to form complex splines, these objects are called nested splines. When extruding a nested spline, the enclosed areas form solid masses and can be used for modeling wall systems and similar geometry.
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This is a short excerpt from the ASCENT Autodesk 3ds Max 2019: Fundamentals learning guide.