In Vitali covering definition i see "derived number" word, but I dont know what that mean.
Example for vitali covering: If $f$ is strictly increasing and
$$E=\{x: \text{ there is a derived number } Df(x)<p \text{ of } f \text{ at } x\}$$
then
$$\mathcal V=\{V \in I: \lambda(f(V))<p\lambda (V) \}$$
forms a Vitali cover for E