The extension CDL is used by the 2D & 3D software CADKey. CADKey was a very popular and commonly used CAD program. In 2003, CADkey was bought by Kubotek, which renamed the software KeyCreator. KeyCreator can handle CADkey files and, according to Kubotek, is the only software that can reliably load, edit and convert all CADKey files.
The last versions (v20 & v21) of CADKey have also saved a new format with the extension ckd. Especially when importing into other 3D programs, problems can occur. The offered demo version of KeyCreator can also be used as a converter. Kubotek Sprectrum Lite also offers a free CADKey viewer.