Augustus of prima porta explain three examples of symbolism used by the artist when designing this statue.

Generally, when a ruler is represented barefoot, it is to indicate that he is of holy descent, i.e. a man sent directly from the gods with divine properties to lead the vast Roman Empire. His shield represents the battle and victory of the Parthians. By his feet is a personification of the god of love, Eros to show that Augustus is a descendant of Aphrodite Genetrix(parent). On the breastplate itself you have the personifications of the captive countries (Gaul and Spain) in the form of two human beings.

The answer is all of the above.