The mapping of a specific IP address to another specific IP address is usually referred to as Destination NAT. FortiOS has a component that is a bit more specialized along this line called a Virtual IP Address, sometimes referred to as a VIP. FortiOS uses a Virtual IP address to map an External IP address to an IP address. This address does not have to be an individual host, it can also be an address range. This mapping can include all TCP/UDP ports or if Port Forwarding is enabled it will only refer to the specific ports configured.

Virtual IP addresses are typically used to NAT external or Public IP addresses to internal or Private IP addresses. Using a Virtual IP address between 2 internal Interfaces made up of Private IP addresses is possible but there is rarely a reason to do so as the 2 networks can just use the IP addresses of the networks without the need for any address translation. Using a Virtual IP address for traffic going from the inside to the Internet is even less likely to be a requirement, but it is supported.

Something that needs to be considered when there are multiple Public IP addresses on the external interface(s) is that when a Virtual IP address is used without Port Forwarding enabled there is a reciprocal effect as far as traffic flow is concerned. Normally, on a firewall policy where NAT is enabled, for outgoing traffic the internal address is translated to the Public address that is assigned to the FortiGate, but if there is a Virtual IP address with no port forwarding enabled, then the Internal IP address in the Mapped field would be translated to the IP address configured as the External Address in the VIP settings.

In this case any outbound traffic from 192.168.1.127 will go out on WAN1 with the IP address of 172.12.96.127 as the source IP address.

In terms of actually using the Virtual IP address, they would be using in the security policies in the same places that other addresses would be used, usually as a Destination Address.