Why are filters needed
Filters are a set of rules applied to proxies before entering them into the ZennoProxyChecker database after downloading them from the source. Using filters, you can cut off proxies that are obviously inappropriate. The filter is applied to all sources at the same time. At the moment of downloading proxies from the source, proxies that do not meet the filters are discarded and do not enter the general proxy base. Read more here.
Useful links
Configuring proxy outputting
Filters
Main Window (Sources)