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Your question is not very clear, sincerely. if the product pages that marketing only the USA users iare the only ones existing of those products (aka: there aren’t Uk product pages for those same products), then you must not implement the hreflang. The hreflang is a rel=“alternate”, which means that it must always show an alternative URL to the current one. In the ISA product page it should signal as alternative UK users the UK ones, and viceversa. so, if no UK product page exists, then the hreflang does not have an meaning to exist. regarding your second question, if the USA market is very important for you and you want to market it differently thean you are marketing the UK one, then it may be a good idea to create an USa version of your site.