How to read the SAS proc x12 output Spectral Plot of Table G 0?

[iForecast]

Hi,

I'm using proc x12's automdl function, and the procedure produces a Spectral Plot of Table G 0 for (the logarithm of) the original variable.  The plot is similar to the example plot in p. 2788 of SAS/ETS 13.2 X12 User Guide that is reproduced below.  The Guide doesn't explain how to read the plot below.  So would you explain what the vertical axis "Percent of Range" stand for?  My input data is monthly, not seasonally adjusted.  So should I interpret the horizontal axis readings below as the numbers of seasonal and trading-day cycles per month?  That is, there is a 1-month cycle in the example sales data because the frequency of 1 represents 100% of Range?  How should I interpret the trading-day cycles?  In addition, are these cycles taken into account in forecasting the original series?  Thank you.