What are VR and IR routes, and how do the three-digit and four-digit identifiers affect their classification for military training?

VR and IR routes are Visual and Instrument Routes used by the military for training. VR routes are for visual flight, and IR routes are for instrument flight. Four-digit identifiers are used for routes below 1,500 feet AGL, and three-digit identifiers are for routes at or above 1,500 feet AGL.