This is not the latest version of this documentation, for the one that is upto date, please see the latest version (2.6.9)
By default none of the ports are bridged to Guest LAN (g-br-lan, except guest wifi interfaces that will be covered in the WiFi section):
Network Settings - Guest LAN - Bridge Slaves
If you'd like to use the raw device as a custom interface which usually for a SIP trunk or WAN failover, you can remove a port from the bridge by just clicking on it.