Using EEPROM

ESP8266 Size can be anywhere between 4 and 4096 bytes.
ESP32 Size can be anywhere between 4 and 1984 bytes.

The IAS library uses EEPROM for storing the following things:

• Device Activation Code
• App name
• App version
• 3x SSID & Password
• Device name
• Compile date
• HTTPS Thingerprint (* ESP8266 only)
• Config mode password (* only when set)

And if you are using the addfield() method these fields are also stored in EEPROM in a array structure.
The maximum nr of fields in this array is defined with MAXNUMEXTRAFIELDS in the config.h file.

From where can I use EEPROM?

This depends on how many fields you have added to your app and how big these fields are. Since 2.0.1 you can use the following var to query from where it is safe to use eeprom. Anything after that is yours to use.

IAS.eepFreeFrom;

Serial.print(IAS.eepFreeFrom); // will return a unsigned int eg. 536

The serial monitor will also show you the locations in EEPROM.