When the CDR operates along the tolerance border, its periodic output is made by two periodic signal parts: the tracking the error. When tracking is the larger part, the angle represented by the FM modulation index h of the VCO overlaps (almost) with the tolerance function. When tracking is the smaller part and the errors the larger, the tolerance angle tends to the buffer depth and the modulation index becomes very small. This diagram is for the simplest case -amongst practical cases- of a 1st order type 1 architecture using a bang-bang phase detector.