Stm32f429 usb cdc to pc
When your device enumerates itself as CDC-ACM, you get the benefit of being able to access it as if it was a generic serial port.

The ACM subclass was intended for USB-to-UART converters and therefore the data stream generated at the PC side contains not only the actual characters sent to the VCP, but also the control messages to set baudrate, do handshaking etc.

When there is no physical serial port on the device, you can ignore the control messages (acknowledge and/or return bogus values) because the port setting has no influence on data flowing between PC and the MCU.