40G Certifier Birdirectional versus Unidirectional

What is the difference between single direction fiber testing and bidirectional fiber testing on the Viavi 40G Certifier? Is only one fiber attached for single?

Two fibers are always attached for 40G Certifier autotest fiber testing. When single direction is selected, on the 40G Certifier, Fiber 1 is tested from Local to Remote and Fiber 2 is tested from Remote to Local. If bidirectional is selected then both fibers are tested from both directions and the unit will prompt you to switch the fibers during the test. DO NOT switch the fibers at the connection to the testset. Removing the transmit fiber connection, at the testset, invalidates your reference. Switch the fibers at the connection to the panel.