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Unique codes are required if you're setting up cross-sell items. Cross-sell won't work for two items that share the same options code or both have blank code fields.
If you have products with options data, such as apparel, you must create options codes for each possible combination of options. Once every option has its own code, you can manage the inventory level of each option as a separate item.
Note: Making changes to options in other locations, such as the item and Edit All pages in Store Editor can affect the individual option codes in Catalog Manager. If changes are made, it's recommended to review the individual option codes in Catalog Manager to make sure they are still set up properly.