A read/write string property containing the name of the application or the object that has generated the error.
When a runtime error occurs in your code, the Source property is automatically assigned the string “Microsoft VBScript runtime error.”
If the error occurs in an ActiveX component instantiated by your application, the Source property usually contains the class name or the programmatic identifier of the component that raised the error.
Knowing what type of error has occurred within a program is often of little use if you don’t know where the error was generated. However, if you enhance the standard Source property by adding the name of the procedure, class, property, or method when you raise an error, your debugging time can be cut dramatically.