In SQL Server, what does “SET ANSI_NULLS ON” mean?

Question

The definition says:

When SET ANSI_NULLS is ON, a SELECT statement that uses WHERE column_name = NULL returns zero rows even if there are null values

Answer 1

Set ANSI NULLS OFF will make NULL = NULL comparision return true. EG :

        SET ANSI_NULLS OFF
        select * from sys.tables
        where principal_id = Null

will return some result as displayed below: zcwInvoiceDeliveryType 744547 NULL zcExpenseRptStatusTrack 2099048 NULL ZCVendorPermissions 2840564 NULL ZCWOrgLevelClientFee 4322525 NULL

While this query will not return any results:

        SET ANSI_NULLS ON 
        select * from sys.tables
        where principal_id = Null