CPV code 43123000 designates Tunnelling machinery in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
43123000 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 43120000 (Coal or rock cutters and tunnelling machinery, and boring or sinking machinery).