A Suppression List is an audience list that advertisers share with an ad network in order to enhance targeting and ROI during UA campaigns. There are three general types of suppression lists- static, dynamic, and custom. A static list is a list of all device IDs that previously interacted with the app. A dynamic list allows a tracking solution to dynamically pass on the device IDs that install the app. And a custom list would be used for retargeting campaigns, where ad networks would only target users who already have the app, to re-engage them. There are several benefits of suppression lists such as ad spend optimization, increased scale, and tailored messaging.