Clarify how to correctly set up LDAP Credential on FortiSIEM

I have two questions to ask:

Question 1:

I want to understand when I should enter a value in the NetBIOS/Domain field when I set Credential as LDAP method, and when I don’t need to enter a value in the NetBIOS/Domain field.

In my own LAB environment, I don’t need to enter a value in the NetBIOS/Domain field to test succefully., but in the company environment, I need to enter a value in the NetBIOS/Domain field to test succefully.

How can I judge when I should enter NetBIOS/Domain and when I don’t need to enter it? Which settings of Windows Server should I check?

Question 2:

When will the “Test” function of “External Authentication” fail to authenticate, but when using AD users to log in to FortiSIEM, it will succeed?

Is this related to the NetBIOS/Domain field?

I don't know what's wrong, someone could help to clarify those questions?