What a starting address of Row 0 of Flash ?

SFLASH can be written only row-wise. As first 6 bytes of row 0 can be used for BLE public device address, it is recommended that you write from row 1 onwards. You can use the API CySysSFlashWriteUserRow for this.

If you are not really concerned about the device address which is used for advertising, then you can use Full Row 0 also for your custom data. During production, if you want to give individual address to each device, then that can also be done using SFLASH update tool. 

SFLASH can be used for loading the BLE public device address. See line no: 1557 in CYBLE_eventHandler.c file:

if(CyBle_IsDeviceAddressValid(cyBle_sflashDeviceAddress) != 0u)
            {
                (void) CyBle_SetDeviceAddress(cyBle_sflashDeviceAddress);

                ------------------------------------------------------------------------

            }