it forwards requests based on port and protocol.
if u have an http server running on local ip 126.96.36.199 anyone on the 188.8.131.52/24 network can access the site, no other network can see the server.
now on your ZS setup the virtual servers [assuming you use pppoe as your internet on eth0, your 184.108.40.206/24 is on eth1]
Input Interface:ppp0 IP Address:ANY protocol:TCP Local Port:80 remote IP:220.127.116.11 Remote Port:80
Now anyone that connects to ppp0 can access the http server.