In the cartesian $3$-space such an area element is modelled as a rectangle whose edges are parallel to two of the coordinate axes, and assigned lengths $\delta x$, $\delta y$ or $\delta z$ respectively.
- $\mathbf r \cdot \mathbf n > 0$
where $\cdot$ denotes the dot product.
In the event that $\mathbf r \cdot \mathbf n = 0$, the outward normal may be chosen arbitrarily.
If $B$ is complicated in shape, a redefinition of the term outward normal may be appropriate.
- Results about area elements can be found here.