Joe, it's known as a haulage rating. CNR used this with their steam locomotives. Not sure if any other railroads did as well. Basically every one percent = 1,000 lbs of tractive force. So, 34% would rate the locomotive as able to produce 34,000 lbs of tractive effort needed to pull/haul a train. Hot Water might be able to give a more detailed explanation, but that's essentially what it means.
Rob