There are various applications using a .cdl file extension…
With reference to CADKEY, the .cdl extension represents the CADKEY Macro ( .cdl ) file type/format.
CADKEY was a CAD tool by CADKEY, Inc. (later, Kubotek USA, Inc.) that gave way to its successor, Kubotek’s KeyCreator Direct CAD software, in 2003.
In this context, the .cdl file represents a sequence of CADKEY editing actions recorded as a macro.
CADKEY macros ( .cdl ) were used to work on complex parts to automate standard or repeated actions.
Even with CADKEY obsolete, CADKEY macros ( .cdl ) can still be imported in many CAD systems.