As we know, the Projectiveprojective space P^n represent$\mathbb{P}^n$ represents the functor sending X$X$ to the set of line bundles L$L$ on X$X$ together with a surjection from the trivial vector bundle to L$L$.
My question is, what functor does the Grassmannian Gr(d,n)$Gr(d,n)$ represent? Sending X$X$ to the set of rank-$n$ vector bundles together with a subbundlesub-bundle of rank $d$?
More generally, what functorfunctors do flag varieties represent?