CPV code 50531200 designates Gas appliance maintenance services in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
50531200 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 50531000 (Repair and maintenance services for non-electrical machinery).