A subassembly is an AutoCAD Civil 3D drawing object (AECCSubassembly) that defines the geometry of a component used in a corridor section.
Add subassembly objects to an assembly object to create a road cross section. Through tool palette and tool catalogs, AutoCAD Civil 3D provides preconfigured subassemblies for components such as travel lanes, curbs, side slopes and ditches.
For information on stock subassemblies, see Subassemblies at a Glance.