6109.90.40 - Containing 70 percent or more by weight of silk or silk waste

This classification pertains to knitted or crocheted T-shirts, singlets, tank tops, and similar garments that are primarily composed of silk or silk waste, specifically containing 70 percent or more by weight of these materials. These items fall under Chapter 61, which governs apparel and clothing accessories made through knitting or crocheting processes. The "other textile materials" designation here signifies that the garment's dominant fiber content is not exclusively man-made fibers or wool with long sleeves, but rather silk or silk waste as the defining characteristic.

Distinguishing this category from its siblings, 6109.90.10 covers garments of man-made fibers, and 6109.90.15 specifically addresses wool garments with long sleeves. The sibling category 6109.90.80, labeled "Other," would encompass knitted or crocheted T-shirts and similar items made from textile materials not otherwise specified and not meeting the 70% silk or silk waste threshold. Therefore, 6109.90.40 is precisely for those silk-centric knitwear tops.

The subcategories within 6109.90.40 further refine the classification based on the intended wearer. 6109.90.40.10 is for men's or boys' garments, while 6109.90.40.20 is for women's or girls' garments. This division allows for more granular tracking and statistical reporting of these specific silk knit tops by gender and age group.