I think we are not quite thinking along the same lines.
Here is an example with more detail to clarify.
I have used the ModelScripting package and customised the form adding buttons to call some additional scripts.
If I open the ModelScripting form to edit it (say to add another button) using “Edit Form As”, the selected package is “Database Oracle 9i”. I would have expected “ModelScripting” to be the selected package because this is where the open form currently exists.
This is a fairly simple example.
It gets more complicated where I have extended a TDM form (eg Edit Entity) and put it in my own package. If the selected package was the package containing the form that is being edited I could be sure I was working on the correct form. I could then select the package I want to save it to.