Dear Martin,
Of course you cannot trust any documentation or any variable names!
Nor can you trust verbal advice given by others.
I love the 68' saying 'trust no-one over 30'!
I never do myself, either. Especially not myself.
The only source you can trust is yourself.
RevitLookup can help you very much exploring the Revit database.
That is tedious manual labour, of course.
For a more efficient approach, cobble up a little program.
Simply write an add-in or a macro that iterates over all your walls and creates dictionaries mapping their type ids to lists of walls having that id.
Explore the result, and tell us what you observe.
Thank you!
Cheers,
Jeremy
P.S. We will not trust what you say!
P.P.S. The help file says: Element.GetTypeId Method: Returns the identifier of this element's type. The id of the element's type, or invalid element id if the element cannot have type assigned. Some elements cannot have type assigned, in which case this method returns invalid element id. Example code given in C# and VB...