Explanation:
Box 1: Domain Account
The service startup account defines the Microsoft Windows account in which SQL Server Agent runs and its network permissions. SQL Server Agent runs as a specified user account. You select an account for the SQL Server Agent service by using SQL Server Configuration Manager, where you can choose fromthe following options:
* Built-in account. You can choose from a list of the following built-in Windows service accounts: Local System account.
* This account. Lets you specify the Windows domain account in which the SQL Server Agent service runs.
Box2: Domain users
Microsoft recommends choosing a Windows user account that is not a member of the Windows Administrators group.
Box 3: Managed Service Accounts
When resources external to the SQL Server computer are needed, Microsoft recommends using a Managed Service Account (MSA), configured with the minimum privileges necessary.
Note: A Managed Service Account (MSA) can run services on a computer in a secure and easy to maintain manner, while maintaining the capability to connect to network resources as a specific user principal.
References:https://msdn.microsoft.com/en-us/library/ms191543.aspx