iTunes affiliate program
Affiliate programs offer rewards for each customer brought about by the station linking through to the company's website.
Registering with the iTunes affiliate program allows a site to have 'Download' buttons alongside songs that have been played or are playing now, and for a visitor to purchase the track immediately. It is also a requirement for showing the cover art images, which are obtained from iTunes.
Stations can register for the affiliate program at the Apple website.
Once registered you should be given an Affiliate ID. Head to Site Settings, then Now Playing, and complete the iTunes ID field.
The URLs for the purchase links are generated at the time a station's playout/automation system sends us the artist and track being played.
At the time of writing, there isn't an equivalent affiliate program available for Google Play.