When sourcing yarn, you must prioritize Yarn Count (Ne, Nm, or Tex) to ensure it matches your machinery requirements. For high-end textiles, verify the Fiber Length and Uniformity to prevent pilling and breakage. It is essential to request Colorfastness Reports (ISO 105-B02) and ensure the yarn meets OEKO-TEX Standard 100 for chemical safety, especially for apparel. For functional yarns, confirm specific attributes like Moisture-wicking, Anti-bacterial properties, or UV resistance through third-party lab certifications.