How to distinguish between main light and auxiliary light in studio lighting?

Mainly depends on brightness and area.

You can see that there are two white bars on the left and right, and the area on the left is obviously larger than that on the right. Explain this in general and call it the main light.

Lighting is divided into main light, auxiliary light, contour light, background light, skylight and modeling light.

There are many kinds and different expressions.

Take a look at this sometime. Analysis of excellent works in shooting cases.