What makes a function query count as a match or not?

[Christophe Biocca]

I haven't been able to find what makes a function query count as a match
when used a part of a boolean query with Occur.MUST.
A Term query is simple, if the term is not found, it doesn't count as a
match. What's the equivalent for a function query? A score of zero (or less
than zero, as implied by the source code for explain in lucene's boolean
query?). Something else?