An interface must be plumbed before it can pass traffic between the system and the network. The plumbing process involves associating an interface with a device name. Then, streams are set up so that the interface can be used by the IP protocol. Both physical interfaces and logical interfaces must be plumbed. Interfaces are plumbed either as part of the boot sequence or explicitly, with the appropriate syntax of the ifconfig command.
When you configure an interface during installation, the interface is automatically plumbed. If you decide during installation not to configure the additional interfaces on the system, those interfaces are not plumbed.