"A facility for researching battle formations, increasing the number of monsters that can be grouped together. The farther your research goes, the more monsters you will be able to use in battle at once." ~Sega
Each building level unlocks the "Rear" card slot for another unit. To be able to utilize this third slot, one must construct a Library to increase the total cost per unit, or they must purchase a temporary unit cost upgrade in the shop using CP. For more information, visit the Units page.