Here’s some trivia about the use of the required dc:identifier metadata element in ePubs that have been uploaded to Bookworm to date.
|dc:identifier type||Percentage of total|
Although the ePub specification does not describe a particular identifier format, it is strongly recommended that ePub creators use one of ISBN, UUID or URL.
Note: only UUIDs and ISBNs that begin with urn:uuid or urn:isbn are counted here. Assigning either number without the prefix results in them being filed as “unknown.”