Advertisements need to be identified as well. The advertisement identifier is a number, GUID or any other kind of ID that uniquely determines an advertisement. The site specifies if this advertisement identifier has to be unique across all advertisements (sent by DV), or just unique across all advertisements within a single account.
For bulk interfaces the identifier is always generated by DV. For incremental updates the identifier can be generated by the site or by DV.
DV makes sure that once an object has been advertised all subsequent communication, until the advertisement has been removed, always use the same identifier.
If the identifier is to be generated by DV and the identifier will be shown in the advertisement (as a reference number), DV tries to use some attribute that is known to the advertiser (for example the license plate or the object identifier) and helps the advertiser to quickly find the object in his inventory management system, for example when an interested party contacts the advertiser about the advertisement.
If an advertisement has been removed and will be re-added on a later date, DV tries to reuse the identifier (if these are generated by DV). If this is not desirable, DV can also be setup to always generate new identifiers.