What is the origin of the idiom "below the salt" and when to use it?

The origin of the idiom "below the salt" can be traced back to medieval dining customs. In those times, dining tables of nobility often featured a large salt cellar placed in the center. The salt was a valuable and expensive commodity, and its placement on the table symbolized prestige. It is often used to discuss or critique issues related to social inequality, class distinctions, or perceived inferiority.