The 240km refers to the distance setting. The capability of how far the unit can actually test is dependant on the dynamic range specification of the product. So, for instance, if a product has a dynamic range of 32dB for both 1310nm and 1550nm then the unit will be able to test approximately 80km at 1310nm (1310nm has about 0.4dB loss per km) and about 128km at 1550nm (1550 has about 0.25dB loss per km). This is only if the run of cable has no events in the fiber that might cause loss (like sharp bends, splices and connectors) each event causes loss and this will effect how far the unit can actually test.