I am studying the isomonodromic deformations theory, which leads in the case of a $\mathcal{C}_{0,4}$ Riemann surface to the sixth Painlevé equation.
I read that this equation had a $\tau$-structure, so we can determine a $\tau$ function associated to isomonodromic deformations.
Here is my silly question : what is a $\tau$-structure, why is it important ?