Figure 3-2 shows USB-C billboard for alternate mode with USB hub TUSB8044A.

USB Billboard is a device class to communicate the alternate modes supported to a host system. USB Billboard is a simple USB 2.0 device that is required for devices that support USB Type-C Alternate Modes.

Billboard descriptors are used to advertise the alternate mode capabilities and configurations supported by the device through a BOS descriptor capability.

Upon the successful entry into the alternate mode, there is no need to switch to read the Billboard descriptors. Only upon failure to enter the Alternate mode, the PD controller signals to TUSB8044A to expose the USB Billboard interface through GPIO. USB Billboard is exposed on through the USB 2.0 interface. Billboard is needed here to avoid silent failures for better user experience.