The answer to the question yIH nuq? ("What is a tribble?") would presumably be a
definition or description of a tribble.  This being the case, then, the
answer to the question jarlIj qaq nuq?  ("What is your preferable month?")
would presumably be a definition of "your favorite month." But this is not
what you want to find out by asking your question.  What you really mean
to ask is something like "Of all the months, which one do you prefer?"

If I wrote {nuq 'oH jar'e' DamaSqu'bogh}, then couldn't we have again
the same problem ? Couldn't this sentence be understood as "asking for
the definition of the month which you prefer", instead of asking the
listener to name the month which he prefers ?

nuq 'oH jar'e' DamaSqu'bogh?
jarvam 'oH jar'e' vImaSqu'bogh.

As a noun, nuq just substitutes in for the answer. nuq can't substitute in for the answer when nuq is standing in for a pronoun that is playing a verb-like role. Okrand isn't saying that asking for a definition calls for different grammar than asking for a choice. He's just saying that that particular sentence, jarlIj qaq nuq, is formed in a way that asks for a defintion, where what the speaker really intends is a choice, something that that particular sentence doesn't support.

-- 
SuStel
http://trimboli.name