What is meant by the notation $A^{\times{B}}_{\div{C}}$; where A,B,C are real numbers?
A confirmation of the explanation is given slightly further on where it talks about how the uncertainty is derived:

In equation (4) we use the 0.6 dex scatter of $\log L_{\rm UV}$ around
the $L_{\rm UV}$ versus $L_{\rm [O\ III]}$ relation, as an estimate for the uncertainty
in deriving $L_{\rm bol}$ from $L_{\rm [O\ III]}$.

The term &quot;dex&quot; indicates a base-10 logarithm of a unit, which translates into a factor of $10^{0.6} \approx 4$. So translating the error expressed in the logarithm of the quantity into an error expressed on the quantity itself, this becomes &quot;multiply-or-divide by 4&quot;, which here is rendered $4000^{\times 4}_{\div 4}$.
Pela's excellent answer explicitly goes into detail about what this implies for the probability distribution. I usually prefer notation like $10^{x \pm \sigma}$ which sticks to more familiar notation and explicitly implies something vaguely log-normal rather than normal, although perhaps it gets a bit cramped when asymmetric error bars come into play.