What is the meaning of segment in MTRs? Military Training Routes (MTRs) with no segment above 1,500' AGL shall be identified by four number characters (IR1206/VR1207)

MTRs that include one or more segments above 1,500' AGL shall be identified  with three number characters (IR206/VR207).



What is the meaning of segment in MTRs?

What is the difference between three and four?
A route is how you go from place to place.  A segment is a portion of your route which is characterized by a consistent course/altitude.

A route may have one segment, or many.  An MTR is a Military Training Route.  If no portions of the route exceed 1,500 AGL, then it is given a four-digit identifier.

I'm pretty sure that the three-digit / four-digit decision was arbitrary, but I'm ready to be educated.