I have no idea whether it is intentional or not, but only units that actually "attack" can make use of the flag. If you flag "CD" on any bombardment units (like artillery), it doesn't change the effect of a "bombard" at all. However, if you give a combat unit (with A/D values) a 0-range bomabard ability and the "CD" flag, when it attacks another combat unit in a city, the first time the defender suffers one HP of damage the attacking unit has a chance to destroy city improvements of citizens. For example, if you have modded a swordsman to have 0-range bombard and then selected the "CD" flag, when the swordsman takes off an HP from a spearman defending a city, you might also get the message "The Barracks of X Has Been Destroyed" even as the unit battle continues on -- that is CD in action. I've never seen it work against tile improvements as the editor help file states that it should.