CPV code 66172000 designates Financial transaction processing and clearing-house services in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
66172000 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 66170000 (Financial consultancy, financial transaction processing and clearing-house services).