FortiGate Firewall 4 NAT vs PAT
NAT and PAT firewall Policy
Cental NAt ,Session helper and Session

NAT (Network Address Translation)
uses a pool of public addresses that are mapped one-to-one to the private (or "inside") addresses, keeping the port number intact.

PAT (Port Address Translation)
uses a single outside public address and maps multiple inside addresses to it using different port numbers.