This key should be zero or 1. If the key is 0 the client IP that the BASIC program sees is the real client connecting to the engine. If this key is 1 the client IP is the IP number provided by the proxy server (e.g. Apache proxy module).
Eszter SB Engine can be used as a standalone web server or as an auxiliary application server that happens to communicate using the http protocol. In this second case the original HTTP request arrives to the primary web server (for example the Apache web server) and the request is proxy-ed to the Eszeter SB Engine. In this case the client IP that the engine sees is the IP number of the server where the Apache is running. This is not the real client IP. However some programs use the client IP number. There is a standard way proxies use to report the original client IP address. This is the http header field X-Forwarded-For.
If this key is true then the environment is altered before the BASIC programs are executed to hold the IP number reported in this header field instead of the IP number from the socket.