What does $9(9_9)$ mean?

Question

A book claims that $9(9_9) = 9^{387420489}$.

I've never seen such an expression, and I've been unable to find anything about it on Google...

How is it supposed to be evaulated?

For reference, the name of the book is Pasatiempos curiosos e instructivos and this is the page where the expression appears (it's in Spanish but I can provide a translation if needed):

Answer 1

Since $9^9=387,420,489$, I assume that it’s a way of writing $9^{9^9}$. I don’t read Spanish, but that appears to be a discussion of attempts by Arab mathematicians to write large numbers using only three digits; if that’s the case, we’re looking at a historical special-purpose notation that didn’t survive.

Answer 2

$9_9$ seems to be a strange notation of $9^9$, since $9^9 = 387,420,489$.