Restock limits are based on units. They determine how much inventory you can send to FBA. These limits apply regardless of your IPI score.
Storage limits are based on volume, which is measured in cubic feet. These limits determine how much fulfillment center capacity you can use. You will not be subject to storage volume limits for standard-size, oversize, apparel, or footwear items if you maintain your IPI at the required threshold or above. You can see your current storage usage and potential by expanding Storage volume in your Inventory performance dashboard or Shipping Queue.