What is Capacity in science?

Capacity is the limiting value of the unit of measurement for an item you are considering. Say the item you are considering a railway car that is 50' long, has a volume 5,000 cubic feet, and can haul 200 tons or a maximum of 10 elephants. If you want to haul something equal or less than 50' long you can since it has a capacity of 50'. Can't haul something greater than 50' since it is over capacity. Say you want to haul 5,001 cubic foot of something, nope can't do it, it exceeds the 5,000 cubic foot capacity. How about 199 tons, sure it's less than the 200 ton capacity. Seven elephants sure, 11 elephants, nope it would be over capacity. The capacity of a rope would be a measurement of its strength. If its capacity, load it will take without breaking is 500 pounds, the rope will break with a load of over 500 pounds because it is over capacity. A 5 gallon bucket will hold 5 gallons maximum any more and it will run over. A railway tank car that will hold 2,500 gallons, will hold 2,500 gallons of anything, but that is its capacity, any more and it will run over. Say the same railway tank car also had a 20,000 pound capacity. If 2500 gallons of water weighs 20,000 pounds will the tank car hold the water? Yes both the gallons and weight are under or equal to the tank car capacity. Say you have 2500 gallons of molasses that weighs 22,500 pounds? Nope although the capacity in gallons is within capacity the weight is over capacity. Therefore you couldn't put the molasses in the tank car.