What does the "Reliability" field mean?
This percentage indicates how often this particular proxy server responds to our requests. Our back-end software checks every proxy IP several times a day.
Some checks are successfull - reply is received from proxy, some checks fails - proxy server is down or firewall software blocks our requests.
If you divide number of successfull checks by number of all checks and then multiply it by 100% you'll get the value of reliability of this proxy.