These functional groups remind me of 1985 when Maverick flew through the jet wash and Goose and he had to eject from their F-14 Tomcat.  What does this title mean?  What we are trying to say is that carbonyls can be classified two different ways: ejectable or non-ejectable.  What this means is that sometime when a carbonyl is attacked by a nucleophile the carbonyl will eject one of its substituents before it reduces the carbonyl to an alcohol.  After the group has been ejected, then a second equivalent of nucleophile will reduce the carbonyl to alcohol.

In essence, this means if a carbonyl has an ejectable group on it, a nucleophile will add twice to that carbonyl.  Some examples of ejectable and non-ejectable systems are below:

Some useful examples of this can be found in your textbook. But for more information on this, please see organic chemistry

Thanks alot, and as always, happy reacting.