Glad we've been able to answer some of your questions thus far!

Each advertisement features a different ID.

Each slot is provided with a unique code so you can have several advertisements on a single site with different sales methods, different costs, customers, and more.

Publishers can create as many advertisement slots for a site as they wish.

Is there an automatic buying/bidding option for advertisers? If it isn't, it would be good and some options like slot_id, amount etc... is needed.