When flying from Medan to Xiamen, the time of day you decide to fly out makes little to no difference towards the overall price you’ll pay on tickets.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Medan Kuala Namu Intl Airport to Xiamen Gaoqi Intl Airport flight would often be abbreviated as a KNO to XMN flight.