Display freezers, multidecks, deli cases and many chest freezers are designed with a specific airflow pattern. In an open multideck, it is an air curtain across the front; in an island freezer, a layer of cold air across the top of the stock. The load line marks the highest point stock can safely reach. Above it, products sit outside the cold zone and also disrupt the airflow for everything below.
Overloading is tempting before festivals or promotions, when shops want every product on view. The result is warm top layers, frosted products, more energy use and units that struggle to hold temperature. Products should also not block air grilles at the front or back, and shelves should not be lined with paper or cardboard that interrupts airflow.
A supermarket in Chennai stacking ice cream tubs high above the load line of an island freezer before a weekend sale would likely find soft, refrozen tubs on top by Sunday. Keeping to the line, and restocking more often, protects quality.