A native PHP version is a version from the official repository of an operating system. It is installed with "Apache module" or "PHP-FPM". For more information please refer to the article How to set up a web-server. You can set up alternative PHP versions from the ISPmanager repository:
In Web-server settings→ PHP, you can see the versions for every PHP mode.
Log in to the control panel as Admin, select the PHP version and click Apache module to apply it to all the WWW-domains of the ISPmanager users with the mode "PHP as Apache".
For FastCGI (Nginx + PHP-FPM) the version can be changed in PHP list at the user level or in the user editing from at the admin level. Changes apply to all user's WWW domains using PHP in FastCGI (Nginx + PHP-FPM) mode. This way, the setting is applied to the user.
Learn more under PHP modes.