Bootloader address flash write

1. Hi i am utilizing nrf52840 sdk with ver 17.1.10 s140 and i have the central UART as main app from there we are calling bootloader

2. Before calling bootloader ble dfu we are writing the data in the Flash address 0x3e000 4 bytes

3. After entering into bootloader ble dfu mode the flash address 0x3e000  data is FF 

4.  which flash address i can write so that the value remains  even it enters into ble bootloader dfu 

5. for flash writing i am using fstorage funcions   in the main central uart app

ret_code_t ret = nrf_fstorage_erase(&fstorage, 0x3e000, 1, NULL);


ret_code_t ret1 = nrf_fstorage_write(&fstorage, 0x3e000, &data_wrt, sizeof(data_wrt), NULL);