The default is true. It would be more common to specify this for a dialer rather than a port. If the argument is true, it means that the port only supports half-duplex connections. This only affects bidirectional protocols, and causes them to not do bidirectional transfers. If the device is not named, the port name is taken as the device.
Device names are system dependent.
The speed this port runs at. If a system specifies a speed but no port name, then all ports which match the speed will be tried in order. If the speed is not specified here and is not specified by the system, the natural speed of the port will be used by default. Specify a range of speeds this port can run at. The first number is the minimum speed, the second number is the maximum speed.