Basically, fling designates the "energy" to apply after the finger is released from the screen after having swiped around (i.e. a panning event such as UIPanGestureRecognizer). It's like you swipe and then "throw" the object you have moved around: you get the force at which it has been thrown in the direction.
It is useful for me to apply inertia after a panning event on a scroll view (from my own custom framework, not using UIScrollView). Is there any way to get the equivalent on iOS? I don't really understand how it is calculated. It is not giving good results to use the last translation from the UIPanGestureRecognizer as a value for inertia.
You are looking for velocity(in:), which gives you points per second at the moment of measurement (which should be the moment of the gesture's .ended). What I do is convert this "momentum" into a spring animation's initialVelocity to bring the motion gently to a halt.
Alternatively, depending on what you're trying to do here, you might find UIKit Dynamics a better fit for your use case.