I am not familiar with NC codes. In Massachusetts, a 20% reduction system would refer to the fact that certain types of systems are approved for 20% less area than a standard system, based on improved performance. Similarly, some systems are approved for less groundwater offset (i.e. you can put the system 4 feet above maximum groundwater rather than the standard distance of 5 feet).
This type of thing may be what the county is talking about. However, septic codes are usually local, and the only way to understand what the county wants is to ask the local code enforcement official.