Why are certain permissions hidden in developer.android.com

Question

Android permissions are listed here:

Manifest.permission http://developer.android.com/reference/android/Manifest.permission.html

In AndroidManifest.xml at https://android.googlesource.com/platform/frameworks/base/+/master/core/res/AndroidManifest.xml, there are few extra permissions that are not listed in the Manifest.permission.
These hidden permission come with the @hide in thir description in AndroidManifest.xml.

Why there permissions are hidden?
Can these permissions still be used by applications if it is allowed for third party applications?

Answer 1

Those type of permissions are for core system applications (from manufacturers) and/or low-level enough to be AOSP-specific.

Example:

<!-- @hide Allows low-level access to tun tap driver -->
<permission android:name="android.permission.NET_TUNNELING"
    android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
    android:protectionLevel="signature" />

This permission is reserved for Android's AOSP source code. It falls under SYSTEM_TOOLS permission group and can only be used when signed with the same signature as the OS.

The API team simply decided not to expose this permission, and I doubt anyone (except core system applications) can use them or even declare them be using them -- hidden APIs = private APIs.