USB communications device class (or USB CDC) is a composite Universal Serial Bus device class.
The communications device class is used for computer networking devices akin to a network card, providing an interface for transmitting Ethernet or ATM frames onto some physical media.
It is also used for modems, ISDN, fax machines, and telephony applications for performing regular voice calls.
[1] This class can be used for industrial equipment such as CNC machinery to allow upgrading from older RS-232 serial controllers and robotics, since they can keep software compatibility.
The device attaches to an RS-232 communications line and the operating system on the USB side makes the USB device appear as a traditional RS-232 port.