When you connect to a website, it can see your IP address as the “Remote IP” or “Remote Address”. When you surf through a proxy server, these fields contain the IP address of the proxy server instead of your own IP address. So the website will see the address of the proxy instead of your actual address.
But all non-anonymous proxies usually put the IP addresses of their clients (i.e. of the computers using those proxies) in either of the two following request headers (variables): “HTTP_CLIENT_IP” or “HTTP_X_FORWARDED_FOR_IP” There are no strict standards, so one proxy may be sending the IP with “Client_IP” variable and another with “X_Forwarded_For_IP”. There is not much difference between them but they are never used together – either one or the other.
So, if the proxy you are using is not anonymous, the website will be able to see you true IP address in one of these fields.
Requests that are generated by anonymous servers do not have these fields (that’s what makes them anonymous).
The thing is that you can set A4Proxy to create these fields, and put there “fake” IP addresses (they are generated as random numbers, so they are different for each request). As the result, the web site which you are visiting will “think” that you are visiting it through a non-anonymous proxy server (while in fact it is a truly anonymous server with an additional fake field generated by A4Proxy).
Not all web sites will look for these fields (Client_IP or X_Forwarded_For_IP). However, those which do look for them will definitely be confused as the fake IP address will be different for each request.
It may be a good idea to switch on one of these options (and it is not important which one).
Finally, if you want a particular address to be sent to the website in one of that field, you will need to create a modification for that field on the Browser Options tab, in addition to enabling the appropriate Simulate… option.