Why CDC_Receive_FS is empty on usbd_cdc_if.c?

Hi, I create a new project using cubemx 4.10 with hal 1.8 for USB device. I'm looking for receive function and i found CDC_Receive_FS, but there is almost no code in ther:

/**
* @brief CDC_Receive_FS
* Data received over USB OUT endpoint are sent over CDC interface 
* through this function.
* 
* @note
* This function will block any OUT packet reception on USB endpoint 
* untill exiting this function. If you exit this function before transfer
* is complete on CDC interface (ie. using DMA controller) it will result 
* in receiving more data while previous ones are still not sent.
* 
* @param Buf: Buffer of data to be received
* @param Len: Number of data received (in bytes)
* @retval Result of the operation: USBD_OK if all operations are OK else USBD_FAIL
*/
static int8_t CDC_Receive_FS (uint8_t* Buf, uint32_t *Len)
{
/* USER CODE BEGIN 6 */
return (USBD_OK);
/* USER CODE END 6 */ 
}

/* Define size for the receive and transmit buffer over CDC */
/* It's up to user to redefine and/or remove those define */
#define APP_RX_DATA_SIZE 4
#define APP_TX_DATA_SIZE 4

CDC_Receive_FS

APP_RX_DATA_SIZE

CDC_Receive_FS