What is a score?

[David F.]

If I have a score,

myScore = \score { … }

and I try to pass it into a function that expects music (ly:music?), I get an 
error:

error: wrong type for argument 2.  Expecting music, found #<Score>

So if a score is not a music, what it is it?  Where can I find that object 
described in Lilypond’s documentation?  If I wanted to pass a score into a 
function, how would I specify its type?

And ditto to all of the above for book and bookpart.

Thanks,
David