This read-write integer is a control object: writing a specific port number to it enables that port, while writing -1 enables every port on the gateway at once. It's the SNMP-based equivalent of a 'bring port back into service' command, used by an admin recovering a port that was previously disabled or busied out. For example, setting this object's value to 9 re-enables port 9 after maintenance, without needing to touch any of the other ports.