Upload partial tft file

Hello,
In TJC editor, when we change some text of object (button, textbox…) and upload to device, TJC IDE will not update unchanged resources, that means it can minimize upload size.
Capture serial output I saw TJC using the “whmi-wris” command, but instead of sending all of tft file it just sending from offset 0xC20000 of tft file. Open the tft file we saw the changed text is stored at offset 0xC20000.
Why it can do this? Is there command to specify the address of flash? why can device known that they must write to flash at 0xC20000?
Thanks