USP = Unique Selling Proposition/Point.
An obvious example: Phoenician Casino offers instant payouts – that’s it’s USP. If you’re going to promote Phoenician via the 2nd tier PPCs, then the whole advert should be geared around this. Forget about the bonuses, the number of progressive slots and all the other obvious stuff. Just target the ‘instant payouts’ thing in the ad and you should get a higher quality, better informed visitor that is interested in what you are selling.