What is the importance of Antigonus Monopthalamus and Demetrius assuming the titles of kings? What was the context for doing this? What did kingship mean? What did this mean in terms of Alexanders conquest and the future interstate relations?

What is the importance of Antigonus Monopthalamus and Demetrius assuming the titles of kings? What was the context for doing this? What did kingship mean? What did this mean in terms of Alexanders conquest and the future interstate relations?