Meaning of "FD" in /dev/cu.usbmodemFD211? Normally when I plug a CDC serial USB device, I get a device like /dev/cu.usbmodem1411. A remote user I'm supporting who's having difficulty with a serial device is reporting a device with FD in the name: /dev/cu.usbmodemFD211 (on a Macbook Air of unknown vintage running 10.13.4)?

What's the meaning of the FD? I've done a websearch but turned up nothing, though some people mention a lower-case fd instead. I know the digits just signify the controller, port, etc.
There's no technical consequences due to the device having "FD" in its name. It is "just a name", so the operating system doesn't treat it any differently because it says "FD".

The reason for the different namings is that the driver for the USB serial device decides the name of the device file. You will see different names depending on which driver the user has installed.

For these reasons, it is not possible to generally say what the "FD" stands for - as it depends entirely on which driver, the user has installed. Typically "FD" stands for "file descriptor" or "full duplex" in this context.