Is there a document indicating the various error messages recieved by a PI tag when it is not recieving production values eg. shutdown, io timeout, bad, error, or just stale values?
Digital Error States are pretty much self descriptive, the reasons for the Digital Error State being updated for particular PI Points would be dependent on how the PI Points are being updated. For example, if they're being updated via an interface then you could interrogate the log files or look at the data source for the interface; usually the error states are replicated from the source.
You can see the list of Digital Error States on the PI Server's System Digital State Set (e.g. via PI SMT).
We do have a KB article that lists and explains some of the more common digital error states:
See the section titled "SYSTEM digital states that describe interface and tag statuses"
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