A Storage Deck Upgrade (or SDU) is an upgrade item that can be applied to a character's inventory to increase that character's storage capacity. The different types of SDU that players will interact with include a Backpack SDU and a number of different weapon ammunition SDUs. Some kinds of SDUs are automatically applied to a character upon receiving them, requiring no player interaction. These are the Weapon Equip Slot SDU and the Skill Point SDU.