How Cascading Shadow Maps Work
The Cascading Shadow Map method divides the view frustum into a series of distance-based shadow cascades, each of which with steadily lower resolution as you move farther from the camera.
This means that shadows nearest the player's view have the highest resolution while those farthest away have the lowest.
Beyond the range of the Dynamic Shadow Distance property, the system blends back into static baked shadows. In this way, you can take a scene with baked lighting and smoothly transition it to static baked shadows as you move away from objects.
In the image above, notice that the dynamic shadows (nearer the camera) fade away near yellow line, which is used to designate the the dynamic shadow distance.
Area showing static shadows
Area nearer the camera, showing cascading shadow maps