Apple App Site Association
The AASA (short for apple-app-site-association) is a file that lives on your website and associates your website domain with your native app. In other words, it’s a safe way to prove domain ownership to iOS. With URI schemes, which were the standard way for opening apps on iOS until iOS 9, app developers could register any URI scheme of their liking and iOS, without any verification, would respond to those URI schemes by opening apps. For example, if an indie developer registers the fb:// URI scheme for a test app, there was nothing to stop that, even thoughfb:// is used by the Facebook native app. The AASA file makes Universal Links unique and secure because there is no way for an indie developer to host an AASA file on the facebook.com domain.