This web interface allows you to edit the network interfaces (NICs) that are assigned to a zone. You can also change a zone's operating mode (Default, Bridge). See intended zones here.
The zone mode can be changed by selecting a mode from the dropdown list under the zone name. A zone can operate in two different modes:
Default: The zone accesses one NIC directly
Bridge: All assigned NICs belong to the same network and IPFire acts like a switch between those NICs
Each zone can have one (if the zone is in Default mode) or more (in Bridge mode) NICs assigned. This NIC is either accessed natively or via a VLAN. If the RED zone is in any kind of PPP mode, you can only choose one NIC. All further PPP settings including VLANs (VDSL) can be changed in System -> Dialup.
Configuring three zones, using two NICs and one VLAN is a practical guide on how to configure more zones than the physical Network Interfaces available by taking advantage of a Managed Switch to create a IEEE 802.1Q network VLAN.
The zone configuration allows configuring Spanning Tree Protocol (STP) for bridges. Since it is possible add multiple interfaces to the same bridge, it becomes a danger that loops are being created on the network. STP avoids those by disabling bridge ports when a loop is being detected.