Internal Flash and EEPROM Emulation
Flash is not memory that happens to be non-volatile. It is a device with an asymmetric write model, and every design decision about storing settings on an MCU comes out of that asymmetry: you can clear a bit at any time, cheaply, one word at a time — but you cannot set a bit back to 1 without erasing an entire sector, which takes up to two seconds, during which the processor cannot fetch instructions from the same flash it is executing from.