MDAC 2.5 SDK - ODBC Programmer's Reference
Chapter 6: Connecting to a Data Source or Driver

Default Data Source

The driver may select a data source, called the default data source, in certain cases where the application does not explicitly specify one:

• In a call to SQLConnect where the ServerName argument is a zero-length string, a null pointer, or DEFAULT.
  
  
• In a call to SQLDriverConnect where InConnectionString either specifies DSN=DEFAULT or specifies with the DSN keyword a data source that is not contained in the system information.

It is driver-defined how the default data source is specified. This may involve administrative action and may depend on the user.