In general, any activity that involves working above ground or floor level, where there is a risk of falling from one level to a lower one, is considered work at height, and should be performed wearing proper fall protection equipment. In the UK, Belgium, Germany and Switzerland, certain obligations apply for activities carried out at heights of 2 meters or more, and in The Netherlands this figure is 2,5 meters.
It is not considered work at height an activity that is performed on the level, on a permanent staircase or ladder, and it's not treated as fall from height a slip or trip on the level. However, if someone slips from a workstation that's 3 meters above floor level, then that is classified as falling from height.
Although falling from a height of 2-3 meters doesn't sound life-threatening, it can actually lead to serious consequences, as this kind of accidents can be disabling and even deadly. In the European Union there were 521 deaths caused by falls from height in the workplace, in 2014, 554 deaths in 2015, and 462 deadly falls from height in 2016.
A fall from height happens rapidly and the consequences cannot be minimized in the absence of fall protection equipment. Since Earth gravity is 9.8 m/s2, it takes one second to fall 9.8 meters, and after that the body is accelerated, so after two seconds one is falling 19.6 m/s and so on. Thus, any worker carrying out tasks at a height should wear personal protection equipment (PPE) against falls.