You are correct.
So, on the model, if for example station weight set to 0 = empty (nothing visible) and station weight set to 100 = a visible M117 will show up on the model for anyone...even if they do no have tacpack. Each type of store has it's own value...which allows you to change the loads.
Up to this point Tacpack has not even been part of the picture and anyone flying the model with a station payload set to 100 will be showing a M117 on it.
I'm simplifying a little bit here....but when you then add the tacpack files, you put the station weight values that the model uses into the ini file.
So based on the example above, in the tacpack ini file, you would add the M117 to the list of weapons for that station and give it a token weight of 100 (since that is what the model uses to display the M117).
Now, since we have added the store weight to the tacpack ini, when we release that M117 it vanishes from the model (because the store weight has now changed from 100 to 0)...
and Tacpack then immediately adds its own M117 model in place of the vanished one...which then falls away from the airplane.
You need to think of Tacpack as something separate from the model...since it can work with any aircraft, whether the aircraft model has weapons attached to it or not.
I have successfully added Tacpack to the Piglet OV-10A, Aerosoft OV-10A, Virtavia A-4E/F, Alpha B-57B and Alpha F-105D.....all without modifying the model.
If, for example, someone were to re-compile the Alpha F-105D that I added tacpack to so that if the payload value for each station on the MER was set to 50 (the value can actually be anything the modeler wants) so that it would show M117's on the MER.... then I could modify the tacpack.ini file I made with the same value (50) and it would then work seamlessly.
The VRS Wiki is a bit daunting and is geared more towards developers creating stuff like the SWS and Milviz Phantoms.
What just about everyone else is using is Corrado's freeware TMAP method. Here is the link to the TMAP files and tutorials.
https://onedrive.live.com/?authkey=!AP4nEtqTWxvSE_0&id=4B4745D8612A4C3!827&cid=04B4745D8612A4C3
With it, you can literally add Tacpack to any airplane (or boat) you want.
If I understand correctly so are all weapons hard modelled into the aircraft (they are not stand alone objects added to the model by the software)+ they need to be stand alone weapon models which takes over after weapon release. All new weapons need to be approved by VRS and need to be added to the main TP software in an official update (by VRS). And there is coding of the weapon behavior etc. It is far from load and play I'm afraid as it is in other sims. But could be done.
VRS has a wiki page with all the information.
Anyonw with experience with making a weapon 3D model into a working weapon in TP (with knowledge of the coding of the weapon and interaction with VRS)?