Rated capacitor current
It's usually the ripple current, which is the RMS value of current the capacitor sees. 

On a switchmode regulator it should never be higher than half the output current. If there is a parallel ceramic capacitor that can reduce the ripple current in the electrolytic. 

The ripple current causes heating in the capacitor (since the capacitor is not perfect), with power loss related to the square of the ripple current. It's reasonable to rate it at a realistic operating temperature and at a realistic frequency. Often (not always) that's at the maximum temperature rating of the capacitor and a plausible SMPS frequency such as 100kHz.