Easy..
See the top tool bar, top left side, two blocks with a red chain between them? That is the 'link' button. Press that to 'link' a part to another part.
When you link a part to another, you are making it a 'child' of the part you linked it too. Thus, the part that other parts are linked too become the 'parent'.
What you want to do is attach all parent parts to 'exterior' using the 'link' button.
Now, if you have an animated door, you have the inner door, door windows, etc. So that these parts move 'with' your door, you 'link' them to the door. Then they will animate 'with' the parent door. Now, link the door to the exterior. Done. When you look at the parts list with all the trees visible (parents and children branches), you should have all parts inevitably linked to exterior so that the main 'exterior' parent is on the far left side of the parts list, and no other parts are on the far left except that one, (exterior).
Next, when you clone the aircraft for the VC, you change the name of the 'other' exterior part to 'interior' and make sure that 'exterior' and 'interior' are the 'only' parts on the far left side. "Only" those.....
You are good to go then...
If you have a part that isnt linked (for FS2004 exports only) and you have say your inteior, exterior, and one or more parts 'also' on the far left, then your craft 'shouldnt' export. (Export failure).
I hope that helps. Youve now been told about the Birds and the Bees in the Gmax world, and how 'children' come about.
Bill