DDP stands for "Disc Description Protocol", a proprietary format developed by DCA, Inc. DDP files are used when sending music to a CD manufacturer.
Technical details
A DDP 2.0 file set is an industry standard replication format typically prepared by a professional mastering engineer using specialized software. The “file set” typically consists of 4-5 files that contain the audio, IRSC, Track ID information, and optionally, CD-Text information.