Arithmetic Density is the total number of people living in a certain area! Geographers can use Arithmetic Density to create a map of the population of the world or they can choose an area to work with!
Physiological Density is the amount of people per unit area of arable land! Geographers can use Physiological Density to determine how many farmers are present or needed in an area depending on technological advancements!
Agricultural Density is the ratio of the number of farmers to the total amount of land suitable to for agriculture! Geographers can use this to get a basic idea of how much produce we're getting from each farmer, therefore they might create a map of the amount of farmers in each state in the U.S, or they might want to keep track of how much produce each grocery store is getting in an area!