Let $Fl_G$ be the affine flag variety for a reductive algebraic group $G$. While this is not really a variety, I read that we can describe it as a functor: for a scheme $S$, the set $\operatorname{Hom}(S, Fl_G)$ consists of triples $(F_G, \beta, \epsilon)$, where $F_G$ is a principal $G$-bundle on the affine line and $\beta$ is a trivialization away from the point $0$ (as in the definition of the affine Grassmannian $Gr$), and $\epsilon$ is a reduction of $F_G|_{D_0}$ to $B$, where $B$ is a fixed Borel subgroup of $G$.
I could not understand the meaning of "reduction of $F_G|_{D_0}$ to $B$". What does it mean?
