There is selectivity between two circuit-breakers in series when, owing to an overcurrent flowing through both of them, the load-side circuit-breaker opens thereby protecting the circuit, whereas the one on the supply-side remains closed, thus guaranteeing power supply to the rest of the installation.
Selectivity for any overcurrent value there may be in the installation.
Selectivity only up to a certain Is current value (ultimate selectivity value). If the current exceeds this value, selectivity between the two circuit-breakers is no longer guaranteed.