The 9900-level code is a supplemental classification that modifies duty treatment for goods already classified in Chapters 1–97. Both the primary commodity code and the applicable 9900-series subheading must appear on the import entry; the Chapter 99 code alone does not describe the merchandise for admissibility or statistical purposes.

When USTR or another agency grants a product-specific exclusion from a Section 301 or Section 232 measure, the exclusion is typically implemented by adding a new Chapter 99 subheading or by specifying that a particular subheading does not apply to the excluded product. Importers must reference the exclusion's Federal Register notice and confirm the correct Chapter 99 treatment with their customs broker before filing.