Carbon-14 cannot be used to date biological artifacts of organisms that did not get their carbon dioxide from the air.
For example if you have a fossil trilobite and it was found in the Wheeler Formation.
Relative age dating also means paying attention to crosscutting relationships.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
The carbon-14 it contained at the time of death decays over a long period of time.
By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.