USP stands for United States Pharmacopeia, quasi the Encyclopedia for pharmaceutical products issued once per year by the United States Pharmacopeial Convention, or rather maintained and updated.

This USP is a long list of medications, ingredients, nutritional supplements for human consumption, their standard, strenghth, quality, purity and production methods of which are specified and described therein. In a way it's something for pharmaceutical products equivalent to the Encyclopædia Britannica - remember that voluminous running metre of books, bound in sleeves of pigs' leather with gold print? It used to exist in a prominently visible shelf in academic households in the last century. Today rather obsolete thanks to Wikipedia and thousands of other information sites in the worldwideweb.

So if a producer asserts that his/her product or ingredients therein were conforming to USP standard, then that has to be so - since it will have legal implications, at least within the USA, should it turn out, that this assertion be untrue. In this case the US FDA (Food and Drug Administration) and/or other US Government Agencies would spring into action swiftly.