The "C" is for compensated. It has ports in the barrel/slide to direct some of the gasses upward. IMHO the 9mm does not have sufficient recoil to be of concern. The 9mm does not have sufficient volume of gas to make a real difference. Probably the only Glock that would truly benefit from the ports is the 10mm's.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball