The Reference Code is a unique code used to identify a Trackable.
A reference code, also referred to as a reference number, is a unique code assigned to a trackable. While many people may have a particular version of a trackable, each unique tracking code is assigned a unique reference code. Unlike tracking codes, a reference code is not stamped onto the trackable itself and cannot be used to log a trackable.
Trackable EtiquetteEditIf a user were to be told a tracking code, that user could log a trackable even if they never came in contact with the item. Armchair logging such as this has resulted in trackables being locked. Geocaching.com has now provided the ability for cachers to lock their own trackables as well. Instead, users can use an item's reference code in cache logs, blogs, forums, etc. Anyone can search a reference number and land on the trackable page, but they cannot log the trackable.
In any form, be it writing or media, users should always use the reference code when they identify an item instead of the tracking code. Many geocachers consider this proper etiquette.
You can find this code on the item's page above the "Current Goal".