CPV code 51900000 designates Installation services of guidance and control systems in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
51900000 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 51000000 (Installation services (except software)).