The debugger implements a composite USB device that includes a standard Communications Device Class (CDC) interface, which appears on the host as a Virtual COM Port. The CDC can be used to stream arbitrary data in both directions between the host and the target: characters sent from the host will appear in UART form on the CDC TX pin, and UART characters sent into the CDC RX pin will be sent back to the host.
On Linux machines, the CDC will enumerate and appear as /dev/ttyACM#.
On MAC machines, the CDC will enumerate and appear as /dev/tty.usbmodem#. Depending on which terminal program is used, it will appear in the available list of modems as usbmodem#.