Order in Keysight ADS HB Simulatio.

Dear all,

1. What does the 'Order' in the following Figure represent in ADS HB simulation?

2. It is 5 by default, do we have to change it ? If yes, to what value?

Depends..
If the any waveform or signal has high level of harmonics, it should be increased in order to see all.Highly nonlinear circuits, switched mode Power Amplifiers,Frequency Dividers etc. have harmonics much more than 5.The harmonic number may also improve the accuracy of Phase Noise simulations.( particular case )
Try and see, if you see an improvement over default harmonic level, you can increase and it does not much simulation time.