Above all things, you have to know what port forwarding/port mapping is. Port forwarding refers to the mechanism of packets forwarding. Its essence is firewall's configuration strategy. But its function mainly focuses on packets forwarding, in order to achieve the purpose of operating specific packages (IP address parsing and filtering based on destination port number). As a consequence, it is generally taken out as an independent concept.
If you are using dual or more displays, you are likely to encounter the following challenges:
How to set different wallpapers or screen savers for two or multiple monitors
How to make the same wallpaper display in a perfect way on monitors with different resolutions
How to make sure each display has its own customized window, taskbar, etc.