To which layer can geologists apply the principle of faunal succession to determine the age of the layer?

The principle of faunal succession allows us to determine the age of a given rock layer. This principle is based on the idea that fossils of fauna (animals of a given habitat) and flora can be found be in sedimentary rocks in a particular time pattern. In simple terms, an older fossil will be found in a deeper layer as compared to a younger fossil. Also, these layers could be spread over a large area or countries. The age of the fossil will allow us to determine the age of the rock layer.
To determine the age of the layer, index fossils are used. The index fossils are from the organisms that are common, easily identified, and were found over a large area. In addition, these organisms lived for a specific time interval to allow the geologists to separate them from other index fossils. Some examples of index fossils include pigs and rodents.
Thus, the fossil content will allow the geologists to determine the age of a given rock layer.
I hope this helps.