A military training route is a designated route used by low-level, high-speed military aircraft.
These routes are either designated as either IFR (IR) or VFR (VR) and three or four-letter designations. A four-letter designation is low-level where no segment of the route is above 1,500 AGL. If some segments are above 1,500 AGL, three letter designations are used. All routes are generally below 10,000MSL. The four-letter routes are lower. Some people use the mnemonic “Three in the tree, four on the floor” to help remember.
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge 15-6