I asked Nielsen this question: “Having registered the print version of my book, I’ve now submitted the registration form for the ePub version, but there will also be a mobi version (identical to the ePub) for Amazon. Do I produce another form with the same ISBN?"
This is their answer which I thought would be of interest to some on this forum: "It is completely up to you with regards to registering Kindle editions with Nielsen. If you do register the Kindle editions, they will need new / separate ISBNs. Please note that Nielsen does not send out any data for Kindle editions. This is because Amazon Kindle editions are classed on our database as Retailer Exclusives to Amazon.co.uk. As such the data for these titles is suppressed and not sent out to our data customers, although it will show in your Title Editor account."