A rack oven is a large oven into which entire racks full of sheet pans can be wheeled for baking. Normal baker’s racks hold 8 to 24 full-size sheet pans, but racks made specifically to go into rack ovens usually hold 15 to 20 pans. Rack ovens hold one to four of these racks at once. The ovens are also equipped with steam injectors.
Although this usage is not strictly correct, you may hear the term rack oven used for conventional ovens such as those found in restaurant ranges because the pans are placed on racks rather than directly on the bottom, as in deck ovens.