How to determine the main and auxiliary lights of flash in photography?

The flash that plays the role of the main light source in photography is the main light. The main light line is strong; The flash lamp that functions as an auxiliary light source is an auxiliary light. The light of the auxiliary lamp is very weak.

Because in order to adapt to people's visual habits, it is generally necessary to put the main light on the left side of the subject, but this is not absolute. Judge whether it is the main light source or whether it plays the role of the main light source.