Non-guaranteed inventory is a term used in the context of online advertising to refer to ad inventory that is not sold or reserved in advance, and that is available on a first-come, first-served basis. Non-guaranteed inventory is often sold through real-time bidding (RTB) auctions, where advertisers can compete in real-time to win the right to serve their ads to users. Non-guaranteed inventory can be attractive to advertisers, as it can provide more flexibility and options for ad placements, and can help to ensure that ads are delivered in the most effective and engaging way. Non-guaranteed inventory can also be beneficial for publishers and platforms, as it can help to maximize the value and revenue of their ad inventory, and to provide a better user experience for their audience.