Everything you need to know about how yarn is labeled and certified. https://www.interweave.com/article/knitting/demystifying-yarn-certifications/

The Craft Yarn Council represents the leading yarn companies, accessories manufacturers, magazine, book publishers, and consultants in the yarn industry. There is a lot of information on their site including US vs. UK instructions for knitting and crochet as well as standards for patterns, needles and yarn weights.   https://www.craftyarncouncil.com/