Add subassemblies to a drawing using the subassemblies provided through the Autodesk Civil 3D tool palette or tool catalog, or create your own custom subassemblies.
When using the preconfigured Autodesk Civil 3D subassemblies, adding the subassembly to the drawing creates the subassembly object.
If you want to draw a custom shape and use it as a subassembly, use the Create Subassembly from Polyline feature.
Another approach to creating customized subassemblies is using the Autodesk Subassembly Composer for Autodesk Civil 3D. This method provides you with the ability to create custom subassemblies with simple or complex conditional behavior built in.