Overview: Copy number is the piece identifier. The copy number reflects if the library has one (or more) copies of a single-volume monograph; one (or more) copies of a multi-volume, (e.g. Copy 1, or C.7.). The element is to be not repeatable.
The requirements for this has been updated - https://docs.google.com/spreadsheets/d/1RCZyXUA5rK47wZqfFPbiRM0xnw8WnMCcmlttT7B3VlI/edit#gid=901484405, row # 37.