An NDP is a Non-Distilling Producer. These whiskey producers still follow the strict requirements of making and bottling Bourbon, Rye and other whiskeys, but they purchase it from producers that distill whiskey.
NDPs are required to notate on the bottle that they are not distilling their own whiskey. Often you will find a small notation near the UPC symbol that says Distilled in XXXXX, Produced in XXXXXX. This is very clearly stating that the brand name on the label did not distill the whiskey in the bottle.
Some examples of current expressions that are bottled by NDPs: