This typically happens in a market for inputs where numerous suppliers are competing to sell their product to a small number of (often large and powerful) buyers.
They can also dictate exact specifications to suppliers, for delivery schedules, quality, and (in the case of agricultural products) crop varieties.
[1] Each of the companies runs a series of specialized imprints, which cater to different market segments and often carry the name of formerly independent publishers.
In doing so, they have increased their influence over suppliers—what food is grown and how it is processed and packaged—with impacts reaching deep into the lives and livelihoods of farmers and workers worldwide.
[3] The situation in Australia is a good example since two retailers, Coles and Woolworths control 70% of the national food market.