they are choosing where XL is greater than the resistance to be the inductive part, and similarly where Cparallel Xc is greater than the resistive part ( at that freq) to be the capacitive part.
Note they are assuming a parallel LC ckt where the impedance is maximum at resonance - as you go very high in freq you get just Rac, and very low in freq you get Rdc
The peak in XL is shown - and is interesting - the peak in Xc (curve) is, disappointingly, not shown - but could be calculated.