Water absorption is measured by standardized test methods (such as ISO 10545-3 or ASTM C373) that compare the dry weight of a tile to its saturated weight. The manufacturer's technical data sheet typically states the absorption rate, but CBP may require laboratory testing to verify classification at the correct subheading. Importers should retain test documentation to support their entry.

Yes — heading 6907 is specifically for unglazed tiles. Glazed ceramic flags and wall tiles are classified under heading 6908. If a tile has even a partial glaze or surface coating that constitutes a glaze in the tariff sense, it moves to 6908. The distinction can affect both the applicable duty rate and any trade-remedy exposure, so surface treatment should be confirmed before filing entry.