During the Connect calculation, I²t is derived for checking short circuit faults. This may be a figure you are not expecting.
For devices that are energy limiting, the device data carries an appropriate I²t value for the device size. This is taken from standard data, or provided by manufacturers where appropriate. Cf. BS88 17th edition devices.
For devices that have I²t curve data (For e.g. Eaton Memshield 3 range EMCH type) the short circuit current is applied to the curve, and the value is derived from there.
In other instances, I²t data is derived from Energy squared x disconnection time.