simply put, a business use case (hereinafter BUC) is *everything* your
piece of the business does in response to a business event. In your
example, the business event is "Customer registers interest". This is
a typical business event - it starts outside your business and outside
The BUC is what your business does in response to this: assist the
customer to fill in the form, file the information on the customer,
get one of the back office staff to contact the customer.
That's the BUC. The PUC is whichever parts of the BUC you choose to do
with your product - probably help the customer to fill in the form,
file it and alert the next-in-line staff member.
Unless you automate them, the human activities stay outside the PUC.
I hope this helps.