subworkflow document key

Hi

I know this was asked for here before but never answered, although it appears lots of people are looking for the answer.

Since the Workflow Editor is not scalable, at around 200 elements it crashes even with 4GB of memory, I'm hoping that using subworkflows can solve the scalability problem. However, can someone please explain what a subworkflow “Document key” is? This has been asked all over the web and AX forums but no one seems to know. Microsoft simply says to select the document from the dropdown. There is nothing in the dropdown.

Any help would be appreciated!

Thanks

Jim