LMK00804B output skew and Part-to-Part Skew

Hi Team,

The customer wants to use LMK00804B. He needs to get the mean about output skew and Part-to-Part Skew. 

I can see "calculation for part-to-part skew is the difference between the fastest and slowest tPD across multiple devices, operating at the same
supply voltage, same frequency, same temperature, with equal load conditions, and using the same type of inputs on each device" in the data sheet, but I can not understand the mean.

Can you explain the mean about output skew and Part-to-Part Skew?

Thanks,

Best Regards,