Wat is Impact prefactor?

In the square-root law $I=Y\sigma\,(Q/V)^{\delta}$ with $\delta\approx\tfrac12$, where $I$ is the peak impact as a fraction of the price, $\sigma$ the daily volatility, $Q$ the metaorder’s size and $V$ the daily volume, the impact prefactor $Y$ is the dimensionless constant, of order one, that sets the level.