Telephones
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USB communications device class (or "USB CDC") is a composite Universal Serial Bus device class. It provides you with a single device class, but there may be more than one interfaces implemented such as a custom control interface, data interface, audio and mass storage related interfaces etc.
The communications device class is primarily used for modems, and so-called winmodems (which simply means it can sample the telephone line and emit a signal as raw sampled data). However it also supports ISDN and Fax machines. It also supports plain USB Telephones for performing regular voice calls.
Additionally this device class supports computer networking akin to a network card, providing an interfaces for transmitting Ethernet or ATM frames onto some physical media. Microsoft Windows does not support the networking parts of the USB CDC, instead promoting its own derivative named Microsoft RNDIS, a serialized version of the Microsoft NDIS (Network Driver Interface Specification).
This class is generally implemented in embedded systems like telephones to achieve more than one functionality from the device, so that a phone may be used as a modem, Fax or network port. The data interfaces are generally used to perform bulk data transfer.