The book says, in the theorem about local measure distortion,
let $v: U \rightarrow \mathbb{R}^k$ be continuously differentable, $U$ be open, and $\forall x \in U \ \det Dv(x) \neq 0$, where $Dv(x)$ is Jacobian of $v$ at $x\in U$. $P$ is a parallelepiped with center at $(0, \ldots, 0).$
Then, $$\lim_{r\to0}\frac{\mu_L((v (P(x, r)))}{\mu_L(P(x, r))} = \left|\det Dv(x)\right|.$$
I am confused what could $P(x, r)$ mean. Earlier, a parallelepiped is defined as $P=\langle a_1, b_1\rangle \times \ldots \times \langle a_k, b_k \rangle$, and $\mu_L(P)= \Pi_{i=1}^k(b_i-a_i),$ but definition of $P(x, r)$ isn't given.