The ODBC Programmer’s Reference for ODBC 3 defines how ODBC 3.X drivers should interpret the SQLSetEnvAttr attribute specifications from applications written to either the ODBC 2.X or ODBC 3.X API. The Microsoft® SQL Server™ ODBC driver complies with those rules.
One of the attributes controlled by SQLSetEnvAttr is whether connection pooling is to be used. If connection pooling is used with the SQL Server ODBC driver, the DriverCompletion parameter must be set to SQL_DRIVER_NOPROMPT when connecting with either SQLDriverConnect or SQLConnect.