Since Android 1.0 we can install any application without having to download from Google Play, just have to download the APK and allow security settings to be able to install application from unknown sources. An option that only need to activate once and ready.
With Android or the “Unknown sources” option disappears. Now to install applications from external sources We must authorize the application you downloaded this APK to install it as one permit more.
New configuration from external sources
For example, if you download an APK from the Chrome browser, when you try to install it, you will get a warning informing you that the installation is blocked for security reasons, that you review your configuration from external sources.
Now in the new configuration Install other applications We must authorize which applications may install other applications from external sources. In Settings > notifications and applications > special access access to the configuration of this new special permit.
Thanks to this change the operating system is more secure, because the user has total control over what applications can install other applications. In earlier versions, you authorize that any application can be installed from external sources, so the device was at risk if accidentally download an application that then began to download malware without our permission.