When do people actually use the decimal thousandth?

Answer 1

For example, when expressing a measurement, when specifying amounts in an inventory, or amounts of an ingredient - depending on the unit used. For instance, when measuring the length of something fairly small, you might use millimeters and use integers; or you might use meters and use three decimals instead. Note that for some calculations you might need to convert all measurements to meters, for example. For a more specific example, I was in charge of a computer program that handled the inventory of a company; the amounts of materials delivered allowed four decimal digits, which I found to be enough for our purposes. Similarly, the computer program also made cost calculations - the amount of different materials used to make an item had to be specified; once again, we used four decimals.