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HAS is basically a "middleman" between apps and wallets. So if you want to sign in to an app using your hive account, HAS provides the method for authenticating without giving the app your private keys. This would allow basically any website (even outside of Hive) to implement an authentication using Hive accounts. I think this image (taken from https://docs.hiveauth.com) describes it quite good. A website requests a login or broadcast of a transaction and HAS communicates with your preferred wallet application to sign the transaction:

Thanks for explaining.