An unconnected lookup in Informatica can return only one port’s value. This is usually the port which is marked as ‘R’ in the Ports tab of the lookup transformation
But if there is only one Output port in the lookup transformation and all the other other ports are the ones used in the condition of the lookup, the unconnected lookup by default returns the output port. There is no need of specifying the return port.
However if there are multiple output ports, it is mandatory to specify the return port.