CPV code 51550000 designates Installation services of weapon systems in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
51550000 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 51500000 (Installation services of machinery and equipment).