Let's imagine 3 websites, each displaying A-ads.com banners
CodeAndCoins website
LeGaulois website
Satoshi website
I visit first CodeAndCoins website, (CodeAndCoins website will be credited as unique impression) then I visit LeGaulois website, I see
a-ads.com banner but it doesn't count as unique impression since I credited CodeAndCoins website already.
Then I visit Satoshi website, I will see the banner but it won't be credited because I already credited CodeAndCoins firstly
First come, first serve
It counts unique impression, by considering all websites using A-ads.com
edit: I found this explanation from their FAQ, it may be more easy to understand
*whole advertising network: all websites using a-ads.com banner/text advertisements