The F39-JGR5B-L is a double-ended cable for the transmitter side of a light curtain. It's the cable that connects the TX head back to the control unit — not the receiver side, not a sensor-to-controller patch, but the specific run for the emitter. Out here in the grease, when you're swapping a light curtain on a press line, having the right cable end for the TX side saves you a re-pull through the conduit.
Packaged as a box — loose cable, not on a reel. That matters for panel builders who want to cut and terminate to exact length without unspooling a whole reel. For an MRO planner stocking spares on a shelf, the box format keeps the cable clean and coiled.