In any installation, it is essential to ensure that an optical transmitter at one end is connected to an optical receiver. This matching of the transmit signal (Tx) to the receiving equipment (Rx) at both ends of the optical fiber link is referred to as polarity.
In standard cabling systems, this connection is ensured by single fiber connectors (usually SC or LC). On the other hand using 12 to 96 fibers MTP/MPO connectors in high-density cases is a better solution.
The Key, at the ends of connectors, allows recognizing fibers position. The orientation of Key ensures a right link