The Source property of the Err object is a string expression of the location
at which the error occurred in your application. If an unexpected runtime error occurs, Visual
Basic sets the Err.Source for you. If the error occurred in a standard module, the source will
be set to the project name. If the error occurred in a class module, Visual Basic uses the project
name and the class module name for the source. For errors in class modules, the source will
have the form project.class.
Like the Number and Description properties, the Source property can be set in
code. Even if Visual Basic generates the error and populates the properties of the Err object,
you might want to overlay the source with a more descriptive text. Because Visual Basic
uses only the project, class, and form names to create the source description, you may want
to add to the source to specify a more exact location for the error. You could add the name
of the procedure in which the error occurred, for example.
The Err.Source property is typically used in an error-handling routine within
a procedure or a common error-handling procedure for the application (see the
section "Handling Errors in Code").