Backup domain
The small corner of a microcontroller with its own power supply, which survives both resets and deep sleep.
On STM32 the domain is fed from the VBAT pin, usually wired to a coin cell or a supercapacitor. It houses the real-time clock, its 32.768 kHz oscillator and a handful of backup registers — on some parts a few kilobytes of backup SRAM as well. When main power goes, the clock keeps ticking and the registers keep their contents. To prevent accidental writes, the domain is write-protected until firmware explicitly unlocks it.
The registers hold whatever must outlive a reboot: a flag telling the bootloader to stay in update mode, the cause of the last crash, or a count of how often the watchdog has fired. With the tamper input connected, the contents are wiped if the enclosure is opened, which makes the domain a cheap place to keep keys.