<milestone> (milestone) marca un punto de frontera que separa cada tipo de sección de un texto,
indicado por cambios en el sistema de referencia estándard, donde la sección no es
representada por un elemento estructural. [3.11.3 Milestone Elements]
For this element, the global n attribute indicates the new number or other value for the unit which changes at this
milestone. The special value unnumbered should be used in passages which fall outside the normal numbering scheme, such as
chapter or other headings, poem numbers or titles, etc.
The order in which milestone elements are given at a given point is not normally significant.