As far as I know you are correct@Pingu,
As far as I can ascertain from reading numerous posts the internal 16 pin is only for the engine whereas the 20 pin in the engine bay has access to the whole system.
Update ....
I have just done a quick check on my car -
From the 20 pin connector in the engine bay I can read both the engine and body data
From the 16 pin connector in the driver foot-well I can read the engine data but not the body data.