>This is the URL I'm getting most of the info on SRU:
>http://www.loc.gov/standards/sru/
>
>Is there another one that might explain the diagnostics, and give more
>details?

From that page, follow the 'Index of Specifications' link and there's
further links to the diagnostics pages.

Or:

http://www.loc.gov/standards/sru/diagnostics.html

HTH,

Rob