You can use the SYSTEM_USER function with DEFAULT constraints in the CREATE TABLE and ALTER TABLE statements. You can also use it as any standard function.
If the user name and login name are different, SYSTEM_USER returns the login name.
If the current user is logged in to SQL Server by using Windows Authentication, SYSTEM_USER returns the Windows login identification name in the form: DOMAIN\user_login_name. However, if the current user is logged in to SQL Server by using SQL Server Authentication, SYSTEM_USER returns the SQL Server login identification name, such as WillisJo for a user logged in as WillisJo.
SYSTEM_USER returns the name of the currently executing context. If the EXECUTE AS statement has been used to switch context, SYSTEM_USER returns the name of the impersonated context.