I am learning about what it means for a model to be Stochastic. To do this, I am examining the stochastic SIR model found here: https://en.wikipedia.org/wiki/Gillespie_algorithm (scroll down to the SIR model)
The stochastic SIR model is
$$\begin{align*} \frac{dn_S}{d t} &= -\frac{\alpha n_S n_I}{V} \\ \frac{d n_I}{dt} &= \left ( \frac{\alpha n_S}{V} - \beta\right )n_I\\ \frac{d n_R}{dt} &= \beta n_I\end{align*} $$
where $\alpha, \beta$ are infection rate and and recovery rate respectively. However, I do not know what this $V$ variable represents nor why we are dividing by it. Could anyone please explain this?