Chapter 16 is the major category in the HS classification system for 'preparations of meat, of fish, of crustaceans, molluscs or other aquatic invertebrates,' covering deeply processed aquatic products that have been cooked, cured, smoked, canned, etc. This chapter does not include animal products that are only simply frozen, refrigerated, or fresh (classified under Chapters 2 and 3), nor does it include meat that has only been simply cut or minced (classified under Chapter 2). The core characteristic is that 'processing' has changed the basic nature of the product, making it a ready-to-eat food or one requiring further cooking. Heading 1604 specifically covers 'prepared or preserved fish; caviar and caviar substitutes prepared from fish eggs.' 'Prepared or preserved' refers to fish products that have undergone cooking, frying, smoking, curing, canning, or other treatments, giving them the characteristic of being directly edible or edible after simple heating. This heading includes whole or cut fish, fish fillets, canned fish, etc., but does not include fish that is only frozen, refrigerated, or simply salted (classified under Chapter 3). Tuna products are classified under this heading due to their deep-processed nature. HS code 160414 consists of 6 digits. The first 2 digits '16' represent Chapter 16, i.e., preparations of meat, of fish, of crustaceans, molluscs or other aquatic invertebrates. Digits 3-4 '04' represent heading 1604, i.e., prepared or preserved fish; caviar and caviar substitutes prepared from fish eggs. Digits 5-6 '14' represent subheading 160414, specifically 'tunas, skipjack and bonito (Sarda spp.),' including whole or cut tuna products. Among these, the 5th digit '1' indicates the first subheading series under 1604 (fish products), and the 6th digit '4' further limits it to tunas, skipjack and bonito. This code does not distinguish specific species but covers all prepared or preserved tuna products. Tuna products are classified under 160414 rather than fresh, chilled, or frozen tuna under Chapter 3 (0302, 0303) because they have undergone deep processing such as cooking and canning, changing their original state and becoming ready-to-eat food. They are also not classified under other subheadings under heading 1604 (such as 160411 salmon, 160412 herring) because tuna belongs to the Scombridae family, different from Salmonidae and Clupeidae. Additionally, if tuna is only simply salted or smoked but not fully cooked, it may still be classified under Chapter 3, but commercially common canned tuna, tuna in oil, etc. are all classified under 160414.