Android - What is protected apps in Huawei phones?

I have owned a Huawei phone (Honor 6) until recently so I can speak from personal experience.

Protected apps is a battery saving feature (like stamina mode in Sony phones). Apps not selected as protected apps will stop working once the screen is off and a wait duration. I discovered this when a voice recording app would stop recording after an hour of recording irrespective of the app. However, if the same app is included in the protected apps list, it would record indefinitely till stopped. Same was the case with an automation app that would work as intended only when included in the protected apps list.

A quick Google search from Android Central's review of Huawei 8 confirms the same

And then there's the whole "protected apps" system, which requires you to manually approve each app you want to be able to run when the screen's off.

According to this Stack Overflow question, developers can't include in their manifest asking for inclusion in the protected apps list, but Huawei OS apparently includes popular apps like Tinder and Instagram (in your case).

Scoured web for a full list of such applications but no luck. Writing to Huawei is an option technically but practically, with experience, they don't bother even acknowledging your mail, forget answering.

Thanks to Happy Coder for updating and confirming that Protected apps is found under Battery manager settings.