In the instructions for registered borrowers, the definition has a note that files should have been purged within the past three years.   The question becomes - Does this mean that the count should not include any patrons who have been inactive three years or more?  OR  Does this mean that records can be purged on whatever basis is set locally, but that such a purge should have been done in the past three years?

The consensus answer is that there is "standard" purging requirement with respect to inactivity of individual borrowers.  That is why the Note in the Registered Borrowers definition is expressed as a "should."  That being said, go with the latter of the two choices.   The library's patron file should be purged every three years.  As to the "expiration date" for individual cards, that is still a local decision.