Geoff--
I would be happy to fill you in on the background for this by phone.
I think would be a lot more time-efficient than email.
The key for this field is that it is to communicate the relative supply/demand
balance for electricity
on the link, as measured by a local price. This is a
different concept (and have different values) than something like a utility grid
AC power price at the meter (there are application-layer protocols for
communicating these prices over many hops). So, this is to be only about
the link.
There should be informative text somewhere to provide guidance on
how it is recommended that the PSE and PD use this field. I was planning
on writing such text, drawing on considerable material I have on this topic
available on my web page, once I figure out the location, format, suitable
length, etc.
Thanks,
--Bruce