MCUBoot flash offset


I'm working on the bootloader part of a project. Using nrf Connect, the integrated MCUBoot bootloader, trying to figure out how all the parts fit together. Running into something I can't quite find back in the documentation.

For me FLASH_AREA_OFFSET(image_1) returns 0x70000. Makes sense, because that's how I've defined image 1 in the dts:

slot1_partition: partition@70000 {
    label = "image-1";
    reg = <0x00070000 0x00064000>;

However, when I enable the bootloader (CONFIG_BOOTLOADER_MCUBOOT=y), FLASH_AREA_OFFSET(image_1) returns 0x83000. That's an offset of 0x13000 bytes; makes sense that MCUBoot needs space, but I can't quite figure out how this value is defined, and how it relates to the values in my dts.

Would like to know how this works, because I have a storage partition defined, and i don't want to risk overwriting it with a future upgrade because of a changing offset (and thus location of that storage partition).