<milestone> marks a boundary point separating any kind of section of a text, typically but not
necessarily indicating a point at which some part of a standard reference system changes, where
the change is not represented by a structural element. [3.10.3 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.