Hi all --
In our work with commcare, we have wound up making quite heavy use of the
special data fields in our forms exports that are called "info.caseID"
and "subcase_0.case.@case_id" ...
The unique codes in those fields make a perfect link between a case and the
subcase that might have been created by entries for that case. in our
case, if a client gets an HIV test in form C1, then it creates a subcase to
track that client's HIV treatment in the treatment clinic. So, in our data,
using those fields, we can trace the progress of a client through those
forms in different cases.
Two things:
1-- I'd be interested to hear if any other clients are making use of those
special fields as well, for their own purposes. Anyone else use those?
2-- More important, I am remembering that a Dimagi staffer once expressed
surprise that I was using those fields. That made me think that it is smart
for me to say: Hey Dimagi folks, as time goes on and you wind up refining
your application and making improvements, it is essential to me that you
don't wind up changing the function of those two fields... 
Important
that those fields continue to function as always even if you also introduce
superior case-tracking features. Need to remain backward compatible on this
one, for the data analysis setups that we've developed over here!
Eric