Some of the most useful fossils for dating purposes are very small ones.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
Suppose you find a fossil at one place that cannot be dated using absolute methods.
That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.
Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.