Inrush Current can be substantial when dealing with high current systems using large hold-up capacitors which may require hundreds or thousands of Amps if left uncontrolled. The hot swap controller limits the current at a preset limit, however, there is still an immediate inrush of current (from 0 to 10A or higher) to charge the capacitors up to this limit. In the cases where the nominal limit is high, this can be a problem as this will cause unwanted transients. The Soft Start function will slowly ramp this current limit from ~20% to 100% of the current limit at a rate determined by the Soft Start Capacitor.  The larger the capacitor the slower the inrush current ramp profile.