The desc explicitly states that given an interface which is a port in a bridge, this lists all its sibling interfaces, separated by a single space. This tells you, for a transparent-bridge deployment, which other physical interfaces are grouped into the same bridge as this one, useful for verifying a bridge is wired the way you expect, such as exactly two ports for a simple in-line bridge. e.g. fwNetIfPorts = 'eth3 eth4' for eth3's row, confirming eth3 and eth4 are the two ports of the same bridge.