The address of the row of flash to write is specified in the "Program Data" DFU Command sent by the DFU Host Tool during a FW update. The DFU Host Tool reads the data, address as well as App ID from the .cyacd2 file. During the FW update, the DFU Host Tool also sends a "Set Application Metadata" command. When the "Set Application Metadata" command is received, the Cy_DFU_Continue API will set the appId member of the cy_stc_dfu_params_t struct. You should be able to use the value of appId to confirm which of the applications that was just updated if needed.
Please refer to "AN213924 - PSoC 6 MCU Device Firmware Update Software Development Kit Guide" for details on the cyacd2 file format and DFU Commands.