There are many effects that PostProcessVolume can do, among which the sun spot effect can be achieved.
If Unbound is checked, it means that it is not limited by the size of the "PostProcessVolume" component and is directly applied to the entire world. If not checked, it will only affect the coverage of the "PostProcessVolume" component.
Looking at the sun, you can see the flare effect.
It can also reflect the lighting effect of smooth screens such as metal!