What is the meaning of "إدخال الطعام على الطعام" (introducing food upon food), and is its depiction in buying and selling or in food itself?

Introducing food upon food means eating without hunger and before the digestion of the previous meal. This is prohibited due to its harm to health. This statement has been attributed to Al-Harith bin Kalada and Imam Al-Shafi'i. This phrase has no relation to buying and selling.