The 4Rs are four words for the decisions in nutrient use: source, rate, time, and place. The framework is called 4R nutrient stewardship. It came out of the fertilizer industry and university extension as a way to talk about those four decisions together, instead of treating any one of them as the whole job. The words are right source, right rate, right time, and right place. "Right" here names the decision. It does not, by itself, supply the number.
Source
Source is which product. Urea, UAN, anhydrous ammonia, MAP, DAP, potash, a blend, manure, and compost can all supply nutrients, and they do not behave alike. Some are available quickly. Some need a conversion in the soil first. Some are lost more easily from the surface. The source decides what a prescription is even counting. A rate written for one analysis is a different application from the same rate of another product.
Rate
Rate is how much. It can be one number for the field, a flat rate, or a surface of numbers, a variable rate. Soil tests, yield, removal, and canopy sensors are all used to arrive at the figure. The 4R label does not choose among them. It says that an amount was decided, for a stated reason, rather than left as whatever the spreader was already set to.
Time
Time is when the product is applied relative to the crop and the weather. Fall, spring, sidedress, and at plant are the names used in annual row crops. A nutrient that stays where it is placed can sit through a winter. A nutrient that moves with water is more exposed the longer it waits in the soil before the crop takes it up. Time is also the hours around the pass: a product that needs incorporation is a different application if it sits on the surface than if it is tilled in or watered in the same day.
Place
Place is where the product lands. Broadcast spreads it across the surface. Banding places it in a strip beside or below the seed. Injection puts a gas or a liquid under the surface. Variable rate is a kind of place as well as a kind of rate: the amount is tied to a location inside the field, not only to a method of placement. Section control is a place decision too, in the negative. It is the choice to put nothing on a given strip.
The four decisions are independent enough to name and connected enough that changing one moves the others. A different source can call for a different time. A different place, such as a band instead of a broadcast, can call for a different rate. Variable rate application is the practice people point at when they are talking about rate and place together. It does not answer source or time. Those are the other two words.