The communitcation device class is primarily used and supported by modems, ISDN and also by fax machines.
The USB Communications Device Class (CDC) provides access to COM port devices via USB. This enables communication via modems for mobile networks such as GSM/3G/LTE or cable based network modems. Any device which uses CDC for configuration and data transmission, such as a point-of-sale terminal, can be accessed via the CDC class driver as well.
An example application which uses the API is provided with any shipment or eval software including emUSB-Host. This example displays information about the CDC device in the I/O terminal of the debugger. In addition the application then starts a simple echo server, sending back the received data. Several application samples can also be found on the emUSB-Host sample page.