hi
if you talk about helicopter engines, n2 is the power turbine. 100% will be maintained to have 100% NR. N1 is the compressor, is driven by a starter(during start up) and after that by the comressor turbine(behind combustion chamber, extracts around 60% of the energie). power turbine and compressor turbine a seperated. N2(power turbine) is the"engine output".