[time-nuts] Osc ppt

[quartz55]

I understand it means parts per trillion (10^12), but can someone tell me what the trace is measuring?  Is it an indication of how far off frequency the osc is at incremental times, relative to what the unit thinks is correct by way of PPS (over time)?  Or point me to reading material please.  I'm assuming the object for precise frequency is to get this trace as stable and the smallest variation possible?

Dave