Size 12 (the finest) - For: patterns where you want a finer thread for more definition, in Embroidery, Stitching, Quilting, Crochet, Punch Needle, heavier Tatting.
Size 8 (medium weight) - For: Stitching, Hand Quilting, Appliqué, Crochet, heavier Punch Needle, Rug-Hooking, Bobbin Work