The sole distinction is the synthetic fiber type: SITC 65331 applies specifically to polyester staple fiber blended mainly with cotton, while SITC 65332 covers all other (NES) synthetic staple fibers — such as polypropylene — blended mainly with cotton at the same weight and blend-ratio parameters. If the synthetic component is polyester, 65331 applies; if it is any other synthetic staple fiber not covered by a more specific heading, 65332 is correct.

Yes — the 170 g/m² ceiling is a hard classification boundary. A fabric exceeding that weight, even marginally, would need to be classified under a different SITC heading covering heavier blended synthetic staple fiber fabrics. Importers should obtain GSM test results before entry to avoid misclassification, particularly for fabrics produced near the weight threshold where manufacturing variation could push individual lots above or below 170 g/m².