What is the concept of the "natural rate of unemployment" in relation to the Phillips curve?

According to the Phillips Curve theory, is  Natural Rate of Unemployment (NRU)  is the rate of unemployment when the economy is producing at its potential output (full employment level) and includes joblessness from people transitioning (frictional) or skills mismatch (structural). It is also known as the Non-Accelerating Inflation Rate of Unemployment,  indicating a point where inflation is stable, not accelerating, and the long-run Phillips curve becomes vertical, showing no long-term trade-off between inflation and unemployment .