Question about SequenceMatcher.quick_ratio

Hi, all.

I am one of  translator of Python document.
But I am not good at English well.

I can't understand one sentence in:
http://docs.python.org/library/difflib.html#difflib.SequenceMatcher.quick_ratio

> This isn’t defined beyond that it is an upper bound on ratio(), and is faster to compute.

I see that quick_ratio() calculates upper bound of ratio().
But I can't get what "this isn't defined beyond that" means.
May someone explain this in easy sentence for me?

-- 
INADA Naoki  <[email protected]>
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