Based on https://www.cnbc.com/2020/08/26/facebook-apple-ios-14-could-cut-audience-network-revenue-in-half.html https://www.cnbc.com/2020/08/26/facebook-apple-io

Based on https://www.cnbc.com/2020/08/26/facebook-apple-ios-14-could-cut-audience-network-revenue-in-half.html https://www.cnbc.com/2020/08/26/facebook-apple-ios-14-could-... It seems to be the unique ID used for targeted advertising. iOS 14 requires user opt-in before the IDFA can be used by third-party apps for advertising. This, I believe, is what Facebook is crying foul about.