There is currently an option to enable "Reference Number" as an additional metadata field under Document Properties - our organisation has been using this field to assign each document a unique identifier to prevent duplicates and improve document control.
However, as we cannot make this field a mandatory field we have to manually review and ensure all documents are assigned a reference number.
Unfortunately, the "Reference Number" meta data field is not surfaced in the Document List report, nor as a separate metadata parameter in the APIs so we are unable to access this information nor report on it...