<milestone> (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. [3.11.3 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.