One is a PHP packaged by cPanel where you have absolutely no control over extensions. The alt-php ones you have control over the extensions so you can use select php version look at say PHP 7.1 and set specific extensions for it and save it. Any site that uses alt-php 7.1 would get those extensions and this is a new feature as of a few days ago. That being said our team discovered a bug where if you have too many extensions loaded it will throw a 503 error. We've implemented a fix for this already and it's propagating to all servers. In the next 24 hours roughly all servers alt-php should work as well from the multi php manager page.