I posted this a couple of days ago in the thread which templar42 refers too. They don't call it NOR1 as such when you get your confirmation e-mail.
NOR1 is a marketing tool which Hilton uses to offer paid upgrades at a supposedly discounted rate. When you receive your reservation e-mail confirmation from Hilton, there is often the option of paying for upgrades to better rooms, paying for early check ins or even discounts for eating in the hotel restaurant. None of these paid requests are guaranteed and are subject to availability when you arrive at the hotel. As Diamonds are entitled to upgrades, depending upon availability, they are not very popular, as you could end up agreeing to pay for something to which you are entitled too. Occasionally they do show free upgrades to the next level room as free for Diamonds, but not always.