The Edge appliance is extremely flexible in terms of the NAT options available to administrators.
One-to-One NAT - Is the ability to map an entire IP address from the WAN to LAN network. Example: Map 18.104.22.168 to 192.168.168.10. All ports and protocols are forwarded as if the internal address was directly connected to the Internet.
One-to-Many NAT - This is a port forwarding NAT rule in which only a specific port is forwarded to the internal LAN address. Example: Any web traffic to 22.214.171.124 would be forwarded to 192.168.168.10, another rule could be created to forward all email traffic to 126.96.36.199 to 192.168.168.11 at the same time.
The apply order at the bottom of the page help you setup a primary and a backup. Apply order 1 would be the primary and apply order 2 would be the backup and so on. Remember that O2O and O2M mapping setup as apply order 2 will not work until the port with apply order 1 is down.