Thank you friends for this informations.

I want to detail about this question.

There is an application server which hostname is XXX, and there is an database server which hostname is YYY. When I check SQL Server logins in YYY, I see DOMAIN\XXX$.

So is that meaning, Services running under XXX 's Local System and Network Service can access to SQL Server in YYY? So, If I dont want applications running under Local System / Network Services in XXX access to SQL Server in YYY, I must start these services using specific account. Am I right?

Thanks in advance.