SYSTEM_USER function allows a system-supplied value for the current system username to be inserted into a table when no default value is specified.
Use the SYSTEM_USER niladic function with DEFAULT constraints in either the CREATE TABLE or ALTER TABLE statements, or use as any standard function.
If the current user is logged in to SQL Server using Windows Authentication, SYSTEM_USER returns the Windows 2000 or Windows NT 4.0 login identification name, for example, DOMAIN\user_login_name.
If the current user is logged in to SQL Server using SQL Server Authentication, SYSTEM_USER returns the SQL Server login identification name, for example, sa for a user logged in as sa.
Above example returns current system user name.