Reservation and open auction buys, a tale of two platforms:
In the early days of internet advertising, if you wanted to place an ad on sites like NYTimes.com, TimesOfIndia.com, or TheGuardian.com, you would have to contact the owner of the site and negotiate to buy impressions from them directly. These transactions, known as tag reservations or direct buys, became the standard method for buying and selling digital advertising.
Over time, as the internet grew in popularity, more and more ad inventory became available. To help publishers sell their inventory in a more scalable way and help buyers find the right inventory, new advertising technologies such as programmatic emerged to handle the large volumes of transactions.
While programmatic ad exchanges provided scale, many top publishers decided to keep their premium inventory outside of the exchanges. This meant that some inventory was available on the open market, and some was still bought and sold through tag reservations. For many years this was the status quo, and even today these two models continue to operate separately