CPV code 63511000 designates Organisation of package tours in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
63511000 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 63510000 (Travel agency and similar services).