Once the drift rate has been measured and compensated for correctly, the slave clock offset should remain fairly constant at each Sync interval. Ideally, once an offset is computed and put in place, it is only rarely changed. The offset is applied to the local time value to synchronize the local time with the master's.
Ultimately, the slave clock uses the drift and offset adjusted counter to generate a usable clock signal externally. Through PTP, each slave free-running counter is both frequency and time-of-day locked to the master clock. The slave clock uses the free-running clock and phase information to generate a frequency and phase aligned clock signal traceable to the master clock.