What does it mean when a passenger or freight rake is referred to as a '15/30' load, or a '36 unit' load?

In order to compute the load to be hauled by the locomotive(s), IR personnel use some rules of thumb. An 8-wheeled passenger coach (of any kind) is counted as 2 units, a 4-wheeled wagon as 1 unit, 8-wheeled wagons as 2, 2.5, or 3 units depending on the payload capacity. A 36 unit load for a passenger train, therefore, may refer to 18 coaches each counted as 2 units. '15/30' simply means a 15-coach rake counted as 30 units. In a goods train made of, say, 30 BOXN wagons, the load may be estimated at 61 units (30*2 for the wagons, and 1 unit for the guard van).

Source - IFRCA.org