The Universal Serial Bus Communications Device Class (USB CDC) is a standardized USB protocol, or "class," that allows a USB device to emulate a traditional serial COM port (like an RS-232C port). This is a very useful standard for ensuring backward compatibility with legacy software. Many older instrument control applications were written to communicate with devices through a COM port. When instrument manufacturers began to replace physical RS-232C ports with more modern USB ports, they needed a way for these new instruments to work with existing software without requiring a major rewrite. The USB CDC provides the solution.
When a USB CDC-compliant power supply is connected to a computer via USB, the necessary driver creates a "virtual COM port" on the computer. The legacy control software can then be configured to use this new virtual COM port (e.g., COM5), and it will send and receive data just as if it were connected to a physical serial port. The USB CDC driver and the instrument's firmware handle the translation, encapsulating the serial data and sending it over the USB connection. This allows for a smooth transition from older serial-based control to modern USB hardware.