You must choose an AUTHENTICATION mode for the Database Engine during setup. Windows Authentication mode and mixed-mode are the two options. In Windows Authentication mode, Windows Authentication is enabled while SQL Server Authentication is disabled. Both Windows and SQL Server authentication is supported in mixed mode. Windows Authentication is anytime enabled and can't be TURNED off.

• You must set up a strong password for the built-in SQL Server SYSTEM administrator ACCOUNT if you choose Mixed Mode Authentication during setup. SQL Server Authentication is used by the system administrator account to connect.
• The setup establishes the system administrator account for SQL Server Authentication if you choose Windows Authentication while setting up, but it is disabled. You must enable the system administrator account if you later switch to Mixed Mode Authentication and WANT to utilise it. As a system administrator, any Windows or SQL Server account can be set up. Since the system administrator account is well-known and frequently targeted by unscrupulous individuals, you should only enable it if it is required by your application.