Region 12 is a region in RAM (not flash!), where boot.bin/ldr.bin (ramloader, speaking generally) has to be executed.
Usually it's not needed to read it, only writing is used, and so sometimes reading is not implemented.

On almost all Garmin devices boot.bin contains bootloader and x-loader, and flashes them to regions 5 and 43, when executed.
Don't modify boot.bin and don't xrgn regions 5 and 43. If you stop bootloader or x-loader from being executed (by erasing or flashing a wrong version, for example), your device will be bricked forever.