Chapter 39 is the general chapter for plastics and articles thereof in the HS classification system, covering polymers in primary forms (resins, pellets, powders, solutions, etc.), semi-finished products, and finished plastic products. This chapter divides headings by polymer source and chemical structure, including synthetic resins such as polyethylene, polypropylene, and polystyrene, as well as cellulose and its chemical derivatives, modified natural polymers, etc. To determine whether goods are classified in this chapter, the core criteria are whether the material is plastic (i.e., capable of being formed by molding, extrusion, etc., and capable of softening or flowing under heat or solvent action), and whether it is in primary form. Heading 3912 specifically covers cellulose and its chemical derivatives, not elsewhere specified or included, and must be in primary forms. It includes: cellulose acetate, cellulose nitrate, carboxymethylcellulose and its salts, hydroxypropyl cellulose and other cellulose ethers and cellulose esters, as well as regenerated cellulose (such as viscose) in primary forms. However, it does not include cellulose products already made into plates, sheets, film, strips, or tubes (generally classified in 3913 or 3916-3921), nor cellulose fiber textile materials (Chapters 54-55) or pulp (Chapter 47). Code 391290 consists of 6 digits. The first 2 digits "39" represent the chapter, covering plastics and articles thereof, defining the broad category. Digits 3-4 "12" represent the heading, meaning "cellulose and its chemical derivatives, not elsewhere specified or included," i.e., varieties other than cellulose derivatives that already have specific subheadings such as cellulose acetate and cellulose nitrate. Digits 5-6 "90" represent the subheading, which is the residual subheading under heading 3912, meaning "other," i.e., any cellulose and its derivatives in primary forms that do not fall under specific subheadings such as 3912.11 (cellulose acetate), 3912.20 (cellulose nitrate), 3912.31 (carboxymethylcellulose and its salts), 3912.39 (other cellulose ethers), 3912.40 (cellulose esters), etc., are classified under 391290. Therefore, this code is essentially the final residual position for "cellulosic materials in primary forms." This commodity is classified under 391290 because it is cellulose or a chemical derivative thereof, is in primary form (such as powder, granules, solution, blocks, etc.), and does not meet the descriptions of other specific subheadings under heading 3912. For example, if it is carboxymethylcellulose, it is classified under 391231; if it is cellulose acetate, it is classified under 391211; if it is cellulose nitrate, it is classified under 391220. Only other cellulose derivatives or cellulose not elsewhere specified or included that cannot be classified under the above specific subheadings use 391290. If it has been processed into plates, sheets, film, or other non-primary forms, it should be classified under 3913 or subsequent article headings, not 3912.