Have you ever wondered how stores decide which discounted items to highlight on their marquee?

Which items are best to highlight on storefronts?

Background

Have you ever wondered how stores decide which discounted items to highlight on their marquee? A drug store may choose to highlight a discount on milk, while a gas station may choose to highlight a discount on diet soda. But each of these stores sells thousands of different items; why did they highlight these 2 items?

Discussion Question

How could the use of the concept of elasticity answer this question of which items are best to highlight on storefronts? Please give specific examples in answering the question.