They say the economy is in "full employment" whenever wages and inflation are threatening to lift too quickly.
And they call that level of "full employment" the NAIRU — non-accelerating inflation rate of unemployment.
However, the NAIRU is unobservable.
They can't say for certain where it is.
No matter. Their estimate of the (unobservable) NAIRU is necessary for the calculation that produces their estimate of the economy's potential output, which is also unobservable.

So, you have two very important numbers which impact the quality of life of millions of us, but which are highly theoretical and unobservable.
This is the framework that dominates our economic policymaking.

https://www.abc.net.au/news/2021-12-12/output-gap-and-nairu-and-economists-forecasts/100677514