Let $X/\mathbb{C}$ be an abelian variety, $Y$ be the dual abelian variety, and $P$ be the Poincaré bundle on $X\times Y$. On p.207, Correction to “Sheaves with connection on abelian varieties” (by M. Rothstein), the remark says that $P$ is naturally a $D_{1,X\times Y/X}$-module, or the so-called twisted (relative) $D$-module (Remark on p.206).

But he does not define the sheaf $D_{1,X\times Y/X}$. I cannot find the definition online, neither. Does anyone have an idea about what it is? Your assistance in this matter would be greatly appreciated.