A reference code is the unique identification of a specific volume or bundle.
You need the reference code to apply for a volume or bundle in the reading room of a repository. Moreover, you need the reference code when you want to order copies of documents.
Searching on a reference code will result in a list of all VOC documents in one specific volume or bundle. The descriptions are presented in a chronological order, not in the physical order in which they are bound in the volume.
Note: Many people only refer to a volume or bundle with the code number of the fonds (in some countries called record group or collection), which is often the same code as the code of the inventory (or catalogue), plus the item number listed in the inventory (which is the code number of that specific volume or bundle).
As one volume or bundle may contain dozens or hundreds of documents, when referring to a document never forget to add the code number of the section plus the numbers of the pages to the reference code !