I am not sure. Maybe someone else knows.
It’s for recording camera timing information I think. If you have a camera integrated into a drone that is using a REACH module as a rover and a Reach RS as a base and you want to do PPK on the photos after, the event.pos has a time stamp that allows you to match the camera events (each time it takes a photo) to the position of the drone based on the corrected gps track of your REACH. Then bam, you have georeferenced photos. (Some work involved, but that is the idea)