What does the vertical value mean in the offset factor graph?

The offset of the Follow Path constraint changes from 0 to 1. But if I try to edit its animation graph, it changes from -1 to 1. But what is that value? Is it speed? But even when it is a negative value, it moves, so I guess it is not speed. The curve’s horizontal angle seems to be the speed, because when the angle is steeper, the speed is faster, but why is the value from -1 to 1 instead of something like 0 to 1?

Offset allows you to play with the position of the object on the curve, independently of the normal speed along the curve, this way you can give a Follow Path constraint to several object, with the same curve as Target, but they can begin at different position of the curve, for example train wagons. You can animate this value if needed, in that case it will play on the speed of the object along the curve. Not sure why you have -1 and +1 though, it should be infinite, but maybe share your file?

Thanks. I recreated a new file and added a curve, cube, follow path, bezier, and somehow this time, it was 0 to 1. Maybe that -1 to 1 was a one time fluke. I will ask again, should it happen again.