Billboard Device Class Spec Revision 1.21 and Adopters Agreement

The USB Billboard Device Class definition describes the methods used to communicate the Alternate Modes supported by a Device Container to a host system. This includes string descriptors that can be used to provide support details in a human-readable format. This specification does not describe the functionality/methodology by which the Device Container shall switch to an Alternate Mode. An example of a Device Container that shall support this class is one that supports PCIe over the USB Type-C connector.

Specification
Device Class Specification