In this case 8400 kJ or 2000 kcal indicates the amount energy what an AVERAGE weighing human doing AVERAGE amount of work per day needs to meet his/her basal metabolic rate. As so when daily input of energy is 8400 kJ / 2000 kcal this AVERAGE (wo)man neither gains or loses weight.
There are so many factors influencing this basal metabolic rate that I wont dig in to those. Basically these reference values are VERY vague, since same reference dont apply to man weighing 200kg, doing hard manual labor compared to 45kg weighing woman doing nothing demanding work. But naturally byrocrats need some reference values to be able give some guidelines what food should be offered to students in schools and old people in retirement homes.
Each domain of this reference intake is then calculated or defined against current evidence in literature. Humans need salt, but salt intake of >6g is known to cause CV diseases and hypertension (Evidence for Nutrition Actions). Human needs fat but excessive amount of especially saturated fats leads to heart disease and high cholesterol (Cochrane Heart Group) . Too much protein can affect kidney function in population level (EFSA Panel on Dietetic Products, Nutrition and Allergies). Then the remaining energy intake should come from carbohydrates, maily food substances with low glygemic index (J Am Heart Assoc.
2012,Diabetes Care November 2004) .
So these values are not maximum, they are not minimum but they give the RATIO how you should eat carbohydrates, fat and protein in order to avoid life-style related diseases. The more you weigh, the more you need energy. The harder you work the more you need energy. The more you do strength trainig the more you need proteins. The more you do long duration exercises the more you need carbohydrates.