[ProSites] Where and how does the Advertising work?

Hi

I'm playing around with Pro Sites at the moment and I've enabled the Advertising Module and also added all the settings for it but where and how can I expect it to be shown on the website?

I'm expecting it to show on the subdirectory sites that people sign up to, encouraging them to upgrade, but I dont see anything.

I couldnt find any details on my searches, can anyone help please?