USM stands for Ko Samui Airport, while MXP stands for Milan Malpensa Airport. These codes are often called IATA codes, a set of identifiers established by IATA (International Air Transport Association) that is widely used by airlines and the aviation industry. Bergamo Orio Al Serio Airport (BGY), Milan Linate Airport (LIN), Parma Airport (PMF) are some additional IATA codes for airports related to your trip.