"Adds one slot to the armor being worn by the user, up to a maximum of four slots."
— In-game description

An Add Slot is a unique, consumable item in Phantasy Star Online. When it is used, it will automatically add one additional unit slot to the armor currently worn by the player character, as long as the armor does not already have four slots.

Add Slot can be obtained by all section IDs in the game. They are primarily found within cargo boxes in most areas.