These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Nelson Airport to Paris Charles de Gaulle Airport flight would often be abbreviated as a NSN to CDG flight. Paris Orly Airport (ORY), Paris Beauvais-Tille Airport (BVA), Châlons-en-Champagne Chalons-Vatry Airport (XCR), Paris-Le Bourget Airport (LBG) are other airports near this flight route and their unique identifiers/IATA codes.