What is "constructive mileage"

Mileage \Mile"age\ (m[imac]l"[asl]j; 48), n.

1. An allowance for traveling expenses at a certain rate per mile.

2. Aggregate length or distance in miles; esp., the sum of lengths of tracks or wires of a railroad company, telegraph company, etc. [Written also milage.]

3. The number of miles that a vehicle can travel after consuming a certain quantity of fuel; in the United States, usually expressed in units of miles per gallon; as, smaller cars tend to get better mileage. It is sometimes used as a nmeasure of the energy efficiency of a vehicle.

4. Use, profit or advantage; as, he got a lot of mileage out of one hit record.

   Constructive mileage, a mileage allowed for journeys supposed to be made, but not actually made.
   --Bartlett.