Question of PSS using on NREM (ClusterStates_GetMetrics.m)

Hello.
I find in ClusterStates_GetMetrics.m section Calculate broadbandslowwave metric you using PSS at the frequency range of 4-90Hz as default for NREM slow wave detection.
I have two questions:
1 Why you use this frequency range, have you mentioned this in any research paper?
2 Why you don't use the frequency range 0 - 4 Hz ? I think it's a usual detection frequency range for NREM.

Best Wishes!