Can some one help me to understand what does it mean to say 2-commutative diagram? context is in page 7 of
arxiv.org/pdf/math/0605694.pdf It is the second diagram in that page... I am guessing it means there is a natural equivalence from $V\rightarrow \mathcal{X}\rightarrow \mathcal{N}$ to $V\rightarrow U\rightarrow \mathcal{N}$... I am not sure about the context of arrow inside the diagram.. In this arrow is pointing towards down and in next page arrow is pointing twoards up..