ICAO defines a route segment as
A portion of a route to be flown usually without an intermediate stop, as defined by two consecutive significant points.
In particular it is used to define the start and the end point of the route segment(s).
As every segment is defined 'from' a start point 'to' an end point, it has an implicit direction. This direction should be consistent with the direction of the route. The start point of a route should also be the 'from' point of the first route segment and the end point of the route should be the 'to' point of the last segment.
The implicit direction of a route segment is relevant when specifying the usage restrictions, using the Route segment usage form, as explained in section Route segment usage [Rsu].
Coding Examples
More examples TBD in the scope of the DONLON XML.