Likely the 3.0l TDI emissions fix(es) - what needs to be done depends on the model year. 2013 and later can be brought into compliance with EPA/CARB, 2009-2012 can't (so far and may not ever). Just as for the 2.0l engines, there are differing opinions as to how much the alterations affect performance, mileage, etc.
Good luck in your search. There are quite a few Tregs out there now ("fixed") and prices are at least as good, if not better than, before the diesel scandel broke. If you're not sure whether one has been fixed, put the VIN in
https://www.vwdiesellookup.com/. If at a VW dealer it's fixed; unlikely any others for sale haven't been fixed, too much money left on the table for the previous owner.