EEPROM emulation allows small amounts of data to be stored and persisted even across reset, power down, and user and system firmware flash operations.
Gen 3 Devices (B Series SoM, Tracker SoM, Tracker One, Boron, Argon, and E404X):
On Gen 3 devices (Argon, Boron, B Series SoM, Tracker SoM, and E404X) the EEPROM emulation is stored as a file on the flash file system. Since the data is spread across a large number of flash sectors, flash erase-write cycle limits should not be an issue in general.
P2 and Photon 2 Devices:
The P2 and Photon 2 also uses a file a file on the flash file system for EEPROM emulation.
Gen 2 Devices (E Series, Electron, Photon, and P2; does not include E404X):
On Gen 2 devices (Photon, P1, Electron, and E Series except the E404X) EEPROM emulation allocates a region of the device's built-in Flash memory to act as EEPROM. Unlike "true" EEPROM, flash doesn't suffer from write "wear" with each write to each individual address. Instead, the page suffers wear when it is filled.
Each write containing changed values will add more data to the page until it is full, causing a page erase. When writing unchanged data, there is no flash wear, but there is a penalty in CPU cycles. Try not write to EEPROM every loop() iteration to avoid unnecessary CPU cycle penalties.
Backup RAM may be a better storage solution for quickly changing values, see Backup RAM (SRAM)).
The EEPROM functions can be used to store small amounts of data in Flash that will persist even after the device resets after a deep sleep or is powered off.