Caesar was used as a title).

By the time of Constantine (although it looks like it started well before him) there was often a Caesar and an Augustus in the imperial government (or more than one of each) with the Caesar having basically imperial authority but being somewhat underneath the Augustus or at least nominally junior to him. So before Constantine’s rise to power, his father was a successful general, who was eventually appointed to be a Caesar by Diocletian. After his father’s death, Constantine’s troops (spontaneously, no doubt) proclaimed him to be Augustus and then he went and fought all the other imperial claimants.