What's the difference between "zend_extension" and "extension" in php.ini?

I noticed that some extensions like Opcache can only be activated (added) via zend_extension , as link :

Non-Windows Platforms:

zend_extension=/full/path/to/opcache.so

Windows:

zend_extension=C:\path\to\php_opcache.dll

I wonder if there is any special reason for this?