What does the Flip 4th Axis message means?

When I am generating output code in Sprutcam I get the following message asking for pre processor input:

"Flip 4th Axis Project Say Y\N (Case Sensitive)"

I have always pressed the deafult N, but now that I am working with the 4th axis I wonder what does this mean? Is this like some sort of flipping to transform from Mill coordinates to Lathe coordinates?

I allways wondered if it meant flip the 4th axis to the right side.

I assume that it reverses the rotation of the 4th axis. That could be handy if you're making a part in the 4th axis and want to make a mirror image of it. Or as David says, you could move the 4th axis to the right side of the table.

Frederic