What's a unique click?
A unique click is a click on your ad from a single computer. If everyone who clicks on your ad clicks on it only once, then your unique clicks number will be the same as your clicks number. But sometimes people click on ads more than once.
For example, suppose you got 100 clicks, but one person clicked on your ad 10 times. In this situation, you would have a click count of 100, but your unique click count would account for the fact that one person clicked more than once. Only one of this person's clicks counts as unique, so your unique click count would be 91.
It's very often the case that only a fraction of people who view an ad actually click it, so you should expect your unique click count to be much lower than your unique user count.
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