Meaning of "FD" in /dev/cu.usbmodemFD211?
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.