Static in the Attic provides a very comprehensive answer.
Although in our type of Operation, reference is made first to MEL.
This is because the CDL refers to operational variations and only lists those defects which have them. There thus can be MEL items without CDL reference (think of a single Nav Position Light Bulb u/s), which in turn is why the CDL is contained behind the MEL in the DDG or DDPG.
If an MEL item has remarks starting with (M) it requires engineering to do some things.
If an MEL item has remarks starting with (O) it has an operational effect, thus a matching CDL item.
If it has merely text there is no maintenance to be performed, no operational effects, thus can be recorded and deferred without further action until the repair interval is due as per MEL.