Given two algebraic varieties $X,Y$ with finitely generated class group, such that exist a small modification $\phi : X \rightarrow Y$, i.e. there exists open subsets $U \subset X, V\subset Y$ such that $codim (X-U), codim (Y-V) \geq 2$, $\phi (U) \subset V $ and $\phi |_U : U \rightarrow V$ is an isomorphism.

Then there exists a equivalence relation on algebraic varieties given by $ X \sim Y \Leftrightarrow \text{ exist a small modification } \phi : X\dashrightarrow Y$.

Where i can find material about this relation?

Thanks in advance.