This feature allows you to use the native USB on an ESP32-S3 MCU as a USB Host. This allows you to connect USB devices (clients) to FluidNC. These could be things like pendants and displays.
Devices like the M5Stack Dial, CYD display and Elecrow HMI have USB device ports. You could directly connect them via USB. The advantages are..
The firmware for the existing pendants and displays will need to be modified to use the USB for communication.
By default the native USB port will act as a USB/Serial client device. You can connect it to a host, like a PC. It will connect as a USB CDC (Communications Device Class) virtual serial port. Most computers should have a native driver for this. It is not setup by the config file. This is controlled by the $USBCDC/Enable setting.
Note: Both modes are dynamic features created by firmware. When the ESP32 resets, the ports will go away and come back. This will cause a disconnection on connected devices. Most client devices connecting to host mode should auto-reconnect. When connecting to a PC in CDC mode, the terminal program on the PC may need to be manually reconnected.
You need to set $USBCDC/Enable=Off and have a config section. Internally to FluidNC, the communication looks like a UART, so you need to setup a UART and link it to a channel.
Example
uart3:
usb_host:
baud: 1000000
uart_channel3:
report_interval_ms: 0
uart_num: 3
message_level: Verbose
Detail
baud: The baud does not matter and does not need to match the client setting. This is a virtual serial port and the data moves at the USB speed.