Specifically, it appears in the following definition
$$ \hat\xi_\tau = \inf_\xi \left\{ \xi \in \mathbb{R} \bigg| \sum_{i=1}^n \rho_\tau(Y_i - \xi) = \min! \right\} $$
where $\rho_\tau(.)$ is the objective function for a quantile regression. Unfortunately, Roger Koenker didn't provide any indication anywhere regarding what the symbols he uses in his book mean. I have never seen that anywhere else, so what does it mean?
It appears at the top of page 69 of Quantile Regression (2005), for reference.