These are administrators who manage entire kintone.com.
These administrators configure common settings used in kintone.com such as managing user accounts and configuring the password policy.

Kintone Users & System Administrators are selected from users who have been added to kintone.com.
Changing Kintone Users & System Administrators

This is a special account that is automatically registered by the system when you start using kintone.com.
Administrator is used for specific functions. For this reason, it cannot be used as a user.
What Is the Administrator Account?