https://developer.android.com/guide/topics/permissions/requesting.html

All dangerous Android system permissions belong to permission groups. If the device is running Android 6.0 (API level 23) and the app’s targetSdkVersion is 23 or higher, the following system behavior applies when your app requests a dangerous permission:

• If an app requests a dangerous permission listed in its manifest, and the app does not currently have any permissions in the permission group, the system shows a dialog box to the user describing the permission group that the app wants access to. The dialog box does not describe the specific permission within that group. For example, if an app requests the READ_CONTACTS permission, the system dialog box just says the app needs access to the device’s contacts. If the user grants approval, the system gives the app just the permission it requested.
• If an app requests a dangerous permission listed in its manifest, and the app already has another dangerous permission in the same permission group, the system immediately grants the permission without any interaction with the user. For example, if an app had previously requested and been granted the READ_CONTACTS permission, and it then requests WRITE_CONTACTS, the system immediately grants that permission.

Any permission can belong to a permission group, including normal permissions and permissions defined by your app. However, a permission’s group only affects the user experience if the permission is dangerous. You can ignore the permission group for normal permissions.

If the device is running Android 5.1 (API level 22) or lower, or the app’s targetSdkVersion is 22 or lower, the system asks the user to grant the permissions at install time. Once again, the system just tells the user what permission groups the app needs, not the individual permissions.