Insertion orders
Insertion orders are signed orders for ad placements at a negotiated price.
Direct sales run on insertion orders. Buying moved from orders people negotiated, to auctions, to platform software that sets the bid. In an insertion order, people negotiate the price. In an auction, bidding software bids on each ad view, and where a campaign uses automated bidding, the platform sets each bid.
Google's AdWords was selling search ads by auction by February 2002, and display ad exchanges, marketplaces that auction each ad view to bidding software, followed around 2005.
Example
A 2017 academic survey of the field found that display ads were sold either in real-time auctions or in advance. Those sold in advance still mainly went through direct sales, which run on insertion orders.