Square D Breakers

I have two large commercial Square D Molded Case Circuit Breaker 20 Amp Single pole 277V AC max (also rated for 120V AC)Mounting Style: I-Line Plug-In (designed for commercial/industrial panelboards)Phase Connection: A-PhaseInterrupt Rating: 18,000 Amps (18kA) at 277VProtection: Thermal-Magnetic (Long-time and Instantaneous trip)

And I have two smaller Square D Breakers both single pole 30amp, one is 10kA and the other is 10A/C not sure what the difference is.