Residual capacity of a reverse edge

If for an edge $(u,v)\in E$, $f(u,v)>0$, then the residual capacity of a reverse edge $(v,u)$ is defined as $c_f(v,u)=f(u,v)$

The positive residual capacity of reverse edge $(v,u)$ represents possibility of decreasing the current positive flow $f(u,v)$.