A Diagram Frame element is a rendition of a diagram dropped from the Project Browser into another diagram. It is a type of Combined Fragment with the Interaction Operator ref. However, it can be created on any type of diagram, and is not created in the same way as other Combined Fragments.
When you drop the diagram from the Project Browser onto the open diagram, the following prompt displays:
If you click on the Diagram Frame radio button, a Diagram Frame is inserted into the diagram, containing an image of the dropped diagram.
If you select the Diagram Reference option, an empty frame is inserted with the name of the dropped diagram in the frame label. If you select the Hyperlink radio button, a diagram icon is inserted with no frame, and with the parent package and diagram name next to it.
In all three cases, the object acts as a hyperlink to the real referenced diagram. You can also define properties for the objects, as for other elements, by right-clicking on the object and selecting the element Properties context menu option.