This lecture introduces the concept of reference flux, which represents the products needed to fulfill a given functional unit. Key parameters, such as product lifespan and material quantity, are crucial in determining reference flux. Through examples like shoes and cups, the instructor explains how different scenarios impact reference flux calculations. The comparison between popcorn and polystyrene packaging materials highlights the importance of defining the functional unit correctly. By analyzing the environmental impacts of each material, the lecture emphasizes the significance of considering key parameters when evaluating product sustainability.