It’s a measure of the discharge time over which the battery is rated. C20 is a 20 hour discharge, C10 is a 10 hour discharge, C5 is a 1 hour discharge and so on. For example a 7 AH battery should be able to discharge 0.35 amps over 20 hours. But the same battery will probably deliver only 9 hours of discharge if discharged at 0.7 amps i.e double the current. This ratio delivers only 6.3 AH (0.7 amps * 9 hours)
While purchasing a battery it’s important to check the discharge ratios over different time periods to understand the efficiency of the battery. Any good manufacturer will always provide this data in the battery specifications.
It’s important to note that in real life, most batteries used for UPS & Telecom backup applications are discharged over a period ranging from a few minutes to maybe 4 hours. So the real AH delivered is always less than half to 3/4th’s of the rated capacity at 20 hours. This is one of the reasons why the battery over sizing has to be considered while calculating backup times for a UPS.