A DesignMerge Data Source Definition (DDF) is a construct that describes the data file you are using so that DesignMerge will know how to import data from that file. The DDF holds all of your Variable Link and Rule settings as well. DesignMerge provides two different methods for managing your DDFs: the Document DDF and the Named DDF. For more information, see the Document DDF versus Named DDF, which one to use? knowledgebase article.