Require approved client app vs Require app protection policy

Navishkar Sadheo Good question. 

First of all:  Require approved client app grant control only supports iOS and Android and is used to only allow 1rst party apps to connect to Office 365, like the Outlook app.

Require app protection policy (preview) is all about Intune app protection. So when you select this option, the app cannot be used without an app protection policy in place. This is meant to protect the data from those apps. 

Please read these articles to learn all about that: https://www.petervanderwoude.nl/post/conditional-access-and-requiring-app-protection-policy/

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https://samilamppu.com/2019/07/09/conditional-access-require-app-protection-policy/ 

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https://www.inthecloud247.com/azure-ad-conditional-access-explained-android-and-ios/