As of Aug 2023 this applies to cameras that are synchronized. It is only implemented in spinnaker dual camera. See How to do Synchronized Capture with FLIR Spinnaker cameras?.

After connecting all the wires for sync the black wire is available as an input to openIris. It can be used to record EVERY frame. The status of a line. 0 or 1 depending if voltage is >~ 3.3 v.

You also need to connect the ground to the brown cables.

The status of that line gets saved in the data file under columns

To see live if it is working activate debug and check