What does TDV mean in Audi A4?

Direct from Audi UK, "TDV" stands for Technology Development Vehicle.

It basically denotes that a diesel particulate filter is present on the Audi in question but the vehicle was produced just prior to (or during) the legislation process regarding DPF's. The abbreviation 'DPF' was not yet officially in existence so Audi used the 'TDV' letters to identify the diesel particulate filter.

Explained by a very helpful chap at Audi UK on the 'phone - 22/03/2012.