There are mainly two types of permissions, they are
- Normal Permissions
- Dangerous Permissions
Normal permissions indicates that there's no great risk to the user's privacy or security in letting apps have those permissions. For example, users would reasonably want to know whether an app can read their contact information, so users have to grant this permission explicitly. By contrast, there's no great risk in allowing an app to vibrate the device, so that permission is designated as normal.
Dangerous permissions cover areas where the app wants data or resources that involve the user's private information, or could potentially affect the user's stored data or the operation of other apps. For example, the ability to read the user's contacts is a dangerous permission. If an app declares that it needs a dangerous permission, the user has to explicitly grant the permission to the app.
In this case, SYSTEM_ALERT_WINDOW comes under Normal permissions, that is if an app declares in its manifest that it needs a normal permission, the system automatically grants the app that permission at install time. The system does not prompt the user to grant normal permissions, and users cannot revoke these permissions.
You can see the list of normal permissions in this link and dangerous permissions here.