USB CDC (Communications Device Class) ACM (Abstract Control Model) is a vendor-independent publicly documented protocol that can be used for emulating serial ports over USB.
More information on Wikipedia.
usb_device_class_cdc
Source code: src/drivers/usb/device/class/cdc.h, src/drivers/usb/device/class/cdc.c
Test code: tst/drivers/usb_device/main.c
drivers/usb/device/class/cdc.h