UART0 is dedicated to modem trace output from the nRF93M1. It carries a proprietary binary log stream for use with Nordic's cellular monitor tool or for capture and supply to Nordic as part of technical support.
The default baud rate is 3 Mbit/s and the port supports up to 6 Mbit/s.
See the Modem trace topic in the Debug and trace chapter for runtime port selection and trace configuration.