Going to hit it with a hammer!!

[Alibert]

There is another setting that is directly linked to these two and that is the steps/mm of the extruder. Basically the software calculates how much volume of plastic it needs to lay down, and calculates how many mm of filament it needs to extrude (how many steps it needs) to do this from the diameter and the steps/mm of the extuder setting. The firmware setting may be a tad off from reality.

This is just the first rough estimate. The feedstock multiplier is a fudge-ing parameter to account for a multitude of effects such as material characteristics (viscosity, which is a function of temperature), printing speed etc.

What the extruder actually extrudes is not exactly what the extruder tries to push out as due to the backpressure the teeth of the extruder gear will slip/deform the filament a bit. The latter effect depends on backpressure and thus on viscosity and nozzle diameter, which in turn depends on printing temperature, polymer characteristics and printing speed. Also the squashing pressure of the filament between gear and pulley will have an effect as it deforms the filament which in turn has an effect on the amount of slip.

So at different printing speeds/temperatures you mayl need different feedstock multipliers, especially if you push the speed very high or temperature very low (both increasing the backpressure a lot). At lower speeds/higher temps the feedstock multiplier has less importance. Also, each printer will need a slighty different feedstock multiplier due to the multitude of effects/settings. It is a fine-tuning parameter after all.