User blog comment:Simplicityaboveall/Extremely Large Numbers 2/@comment-1605058-20160801201054

7. The notion of "computable number" and "uncomputable number" is really vague, and in googology it seems like they generally refer to googologisms defined using a, respectively, computable or uncomputable notation (so that the notion of (un)computable number actually refers to not the very number itself, but rather its definition). If we take these definitions, then, for example, $$10^10$$ as defined using exponentiation is a computable number which is larger than an uncomputable number $$\Sigma(4)$$.

8. Rayo's number has a rather slow start, since writing down formal definitions in the language of FOST can take a bit of space. From this discussion it follows that, almost certainly, values of Rayo(n) are undefined for n<10, and Rayo(10)=0.