When xLAN is enabled for a Site, all Appliances in that Site will use internal GRE tunnels to forward local zones and automatically imported remote zones. The AutoVPN appliance acts as the GRE tunnel concentrator. You will only need to use xLAN if you require multiple Zones in a Riverbed Appliance which does not support VLAN operations on the wire.
A typical use case is to enable an Access Point to broadcast SSIDs on different Zones when it isn’t connected to a switch that supports tagged VLANs.