Having an unemployment rate below the natural rate of unemployment (NRU) leads to wage inflation and a reduction in productivity. Seeing the consequences of that, and knowing that one of the roles of a central bank is to control unemployment and inflation, central banks must accurately determine what the unemployment is currently at and see whether it is above or below the NRU.
I ask this because after looking at the Bureau of Labor Statistics' data on U-3 and U-6 (seasonally-adjusted) between the years 2010 and 2020, the lowest U-3 got was 3.5% in 2019Q4, whereas U-6 was 6.5%. However, the NRU in 2019Q4 4.45%
So, what measurement is used by central banks and what would be the best way to measure unemployment so that it can best be compared to the NRU?