To achieve selectivity with down-stream breakers you need both a higher trip current for the thermal curve aspect, but you also need some form of time delay on very high currents so if the fault goes above the thermal overload curve in to the magnetic 'instant' trip region the upstream device holds on for long enough that the downstream one can clear if possible.
So the larger breakers such as ACB and some MCCB (category B) often have a two-stage high current trip. There is often a fixed magnetic point where the breaker goes 'instantly' and tries to limit the fault energy, but below that you often have an electronic adjustable trip point in both short-time delay and related short-time trip current to allow some coordinated selectivity with those further down-stream.
However, having a fixed square curve for trip current and time is not necessarily the best way of getting good enough selectivity while still protecting the supply and cables, etc. So some devices have two curve options, a simple "step" mode where above Isd it holds for Tsd, and a I2t mode where there is a slope approximating the constant I2t sort of region that you might optimise for cable protection, etc. Here is a good overview but usually you need to check with the manufacturer's specifications as they are not all standard like MCB are:
There are six basic adjustable tripping settings (functions) you really should understand in order to fully understand how circuit breaker
electrical-engineering-portal.com
This rather bizare link works for me, seems they don't make it too easy to find:
