USB Bulk Device Data

Hi Everybody !

I had a problem in undersatnding the USB BULK DEVICE EXAMPLE C

int32_t
RxHandler(void *pvCBData, uint32_t ui32Event, uint32_t ui32MsgValue,
          void *pvMsgData)
{
    //
    // Which event are we being sent?
    //
    switch(ui32Event)
    {
        //
        // We are connected to a host and communication is now possible.
        //
        case USB_EVENT_CONNECTED:
        {
            g_bUSBConfigured = true;
            //g_ui32Flags |= COMMAND_STATUS_UPDATE;

            //
            // Flush our buffers.
            //
            USBBufferFlush(&g_sTxBuffer);
            USBBufferFlush(&g_sRxBuffer);

            break;
        }

        //
        // The host has disconnected.
        //
        case USB_EVENT_DISCONNECTED:
        {
            g_bUSBConfigured = false;
            //g_ui32Flags |= COMMAND_STATUS_UPDATE;
            break;
        }

        //
        // A new packet has been received.
        //
        case USB_EVENT_RX_AVAILABLE:
        {
            tUSBDBulkDevice *psDevice;

            //
            // Get a pointer to our instance data from the callback data
            // parameter.
            //
            psDevice = (tUSBDBulkDevice *)pvCBData;

            //
            // Read the new packet and echo it back to the host.
            //
            return(EchoNewDataToHost(psDevice, pvMsgData, ui32MsgValue));
        }

        //
        // Ignore SUSPEND and RESUME for now.
        //
        case USB_EVENT_SUSPEND:
        case USB_EVENT_RESUME:
            break;

        //
        // Ignore all other events and return 0.
        //
        default:
            break;
    }

    return(0);
}

My Question is this " what is the meaning of the following statements of the code"

tUSBDBulkDevice *psDevice;

psDevice = (tUSBDBulkDevice *)pvCBData;  //

where the psDevice points to? whether it is the pointer to tUSBDBulkDevice structure or what is it pointing to ?

and what is the source of data in the code ? how can i modify the source of data for USB Bulk Device Example ?

please help me , i have tried enough but did not get the meaning of the same.

Thanks in advance.

ODE,  here is the code,