6907.21 - Of a water absorption coefficient by weight not exceeding 0.5%:

This HTS classification, 6907.21, pertains to ceramic flags, paving, hearth, and wall tiles, as well as ceramic mosaic cubes and similar items. Specifically, it covers products that are characterized by a very low water absorption coefficient, not exceeding 0.5% by weight. These items are typically dense, vitrified ceramic materials intended for flooring, wall coverings, or decorative mosaic applications.

The key distinguishing factor for this category is its stringent requirement for a water absorption coefficient below 0.5%. This differentiates it from sibling categories such as 6907.22, which covers ceramics with a water absorption coefficient exceeding 0.5% but not exceeding 10%, and 6907.23, which includes ceramics with an absorption coefficient above 10%. The low absorption rate implies a higher degree of vitrification and reduced porosity, contributing to greater durability and resistance to staining and frost.

Within this category, further distinctions are made based on the surface treatment. Subheading 6907.21.10 specifically enumerates unglazed articles, meaning they have not been coated with a glassy glaze. The subsequent subdivisions, under the "Glazed" category, will encompass ceramic tiles and similar articles that have been finished with a glassy coating, impacting their aesthetic appearance and further enhancing their impermeability and ease of cleaning.