When sourcing FDY (Fully Drawn Yarn), you must prioritize Denier (D) and Filament (F) counts, as these determine the yarn's thickness and softness. Key metrics include Tenacity (measured in cN/dtex), which should be high enough to prevent breakage during high-speed weaving, and Elongation at Break, which indicates the yarn's elasticity. Additionally, verify the U% (Evenness) to ensure consistent fabric texture and Boiling Water Shrinkage (BWS), which should typically be within 3% to 7% to maintain dimensional stability after dyeing.