a group container for one or more networks that then sit behind a specific firewall barrier that imposes a specific set of firewall rules that controls traffic flowinto and out of the group (zone).
Zones can always be applied to multiple interfaces in the Network > Interfaces page, even without the use of PortShield groupings. However, these interfaces will not share the same network subnet unless they are grouped using PortShield.
Layer 2 Bridged Mode - limited to two lan interfaces.
To have X1 for the wan and X0, X2, X3 and X4 connected to 4 devices that would form a lan.
This is configured via the Network -> Interfaces area, the LAN interface is configured as normal and the "extra" LAN interfaces are set to the LAN zone, PortShield Switch Mode, and PortShield to X0 (our LAN).
X0 - LAN
X1 - WAN
X2 - Portshield to X0
X3 - Portshield to X0
X4 - Portshield to X0