What is the meaning of segment in MTRs?
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.