8540.99 - Other:

Category 8540.99 encompasses "Other" parts for thermionic, cold cathode, or photocathode tubes. This broad category serves as a residual for components not specifically enumerated elsewhere within the 8540 heading. It includes a diverse range of parts that are integral to the function of vacuum tubes, gas-filled tubes, mercury arc rectifying tubes, and television camera tubes, excluding those specifically designed for cathode-ray tubes.

This classification is distinguished from its sibling, 8540.91, which exclusively covers parts of cathode-ray tubes. Therefore, any component that is not a part of a cathode-ray tube but is a component of another type of tube enumerated in heading 8540 would likely fall under 8540.99. The key differentiator is the specific type of tube the part is designed for; if it's not for a cathode-ray tube, it falls into this "other" category.

The subcategories within 8540.99 further refine this classification. 8540.99.40.00 specifically identifies electron guns and certain radio frequency interaction structures for microwave tubes, providing a detailed breakdown for these particular components. The remaining parts that do not fit these specific subheadings are then classified under 8540.99.80.00, which serves as the ultimate residual for all other qualifying parts not specifically itemized.