Today is another example where the right thing to do is not to answer the customer's question but rather to solve the customer's problem.
A customer liaison asked, "What do the registry keys X and Y do? We noticed that they are both read from and written to when you open a common file dialog. Just curious."
I replied, "I'm curious as to your curiousity. I'm afraid that you are curious because your customer is curious, and then the customer will start relying on the keys in a strange and unsupported way." The format of those keys has varied from one version of Windows to another, so there is nothing there applications can rely on. But maybe we can figure out why the customer is snooping there so we can solve the customer's problem.
The customer liaison was kind enough to explain. "The customer wants to know how to set the default folder shown in the Open and Save As dialogs."
The algorithm for choosing the default folder shown in the Open and Save As dialogs is spelled out in the documentation of the
OPENFILENAME structure. There is no registry key that can force all dialogs to use a particular folder. But what is the customer's actual problem?
The customer liaison explained, "The customer wants to change the default save location for Paint and Notepad."
Okay, now we're getting closer to a solvable problem. Paint defaults to saving in the user's Pictures folder, and Notepad defaults to saving in the user's Documents folder, so you can use folder redirection to point the Pictures and Documents folders to locations of your choosing, noting of course that this will affect all applications which look for those folders.
It turns out that this was what the customer actually needed. They redirected the user's Pictures and Documents folders to their preferred location via GPO, and everybody was happy.