An arbitrarily shaped, thin wire through which an electric current $$I$$ flows. The position vector $$\boldsymbol{R}$$ points to the infinitesimal line element$$\text{d}\boldsymbol{s}$$ along the wire. The position vector $$\boldsymbol{r}$$ on the other hand, points to the field point where the magnetic field $$\boldsymbol{B}(\boldsymbol{r})$$ needs to be calculated with the Biot-Savart law. The connecting vector $$\boldsymbol{r} - \boldsymbol{R}$$ points from the line element to the field point.