SourceObject Property [Access 2003 VBA Language Reference]
You can use the SourceObject property to identify the form or report that is the source of the subform or subreport on a form or report. You can also use this property for linked unbound object frames to determine the complete path and file name of the file that contains the data linked to the object frame. Read/write String.
expression Required. An expression that returns one of the objects in the Applies To list.
Enter the name of the form or report that is the source of the subform or subreport in the control's property sheet. If you add a subform or subreport to the form or report by dragging it from the Database window, the SourceObject property is set automatically in the property sheet.
For unbound object frames, the SourceObject property is set automatically when you use the Object command on the Insert menu to insert a linked OLE object.
In Visual Basic, you set this property by using a string expression that is a name of a form or report.
For linked unbound object frames, the SourceObject property can't be set in any view.
If you delete the SourceObject property setting in the property sheet for a subform or subreport, the control remains on the form but is no longer bound to the source form or report.
The following example displays the name of the form that is the source of the ProductList subform control in the Debug window.
Debug.Print Forms!Categories! _ [Product List].SourceObject