Each equippable item has two core ways to increase its stats. Main stats are increased by powering the item up and sub stats are increased by augmenting the item. Item augmentation can be achieved from the same window as the power up. When augmenting an item you are required to use other items as food. Each food item gives the targeted item for augmentation XP depending on the food tier and rarity.
Items have 4 augmentations levels. Each subsequent augmentation level requires more augmentation XP. When an augmentation level is reached the item either gains a new sub stat (Items gain substats until 3 substats are obtained.) or increases a sub stats value.
Example: Worn chest in the image below would gain a substat on the next item augmentation. That sub stat could be either (ATK, RES, HP, ENREG). For the second augmentation he would again gain a sub stat. On the third augmentation the item would increase an existing sub stat.
Item sub stats have a MIN and MAX. Each new augmentation level rolls for its stats randomly.