These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Perth Airport to Nashville Intl Airport flight would often be abbreviated as a PER to BNA flight. Memphis Airport (MEM), Knoxville McGhee Tyson Airport (TYS), Chattanooga Lovell Field Airport (CHA), Blountville Tri Cities TN/VA Airport (TRI), Jackson Mckellar Field Airport (MKL) are other airports near this flight route and their unique identifiers/IATA codes.