Security Types

Each zone has a security type, which defines the level of trust given to that zone. There are six security types:

Trusted: Trusted is a security type that provides the highest level of trust—meaning that the least amount of scrutiny is applied to traffic coming from trusted zones. Trusted security can be thought of as being on the LAN (protected) side of the security appliance. The LAN zone is always Trusted.
Management: The Management security type is unique to the MGMT zone and MGMT interface, and also provides the highest level of trust.
Encrypted: Encrypted is a security type used exclusively by the VPN and SSLVPN zones. All traffic to and from an Encrypted zone is encrypted.
Public: A Public security type offers a higher level of trust than an Untrusted zone, but a lower level of trust than a Trusted zone. Public zones can be thought of as being a secure area between the LAN (protected) side of the security appliance and the WAN (unprotected) side. The DMZ, for example, is a Public zone because traffic flows from it to both the LAN and the WAN. By default traffic from DMZ to LAN is denied. But traffic from LAN to ANY is allowed. This means only LAN initiated connections will have traffic between DMZ and LAN. The DMZ will only have default access to the WAN, not the LAN.
Untrusted: The Untrusted security type represents the lowest level of trust. It is used by both the WAN and the virtual Multicast zone. An Untrusted zone can be thought of as being on the WAN (unprotected) side of the security appliance.By default, traffic from Untrusted zones is not permitted to enter any other zone type without explicit rules, but traffic from every other zone type is permitted to Untrusted zones.