Duration % of Original % Threshold

Hi all P6 Users,

Can any one Explain me that how P6 Calculates Origional Duration and Actual Duration.I was applying a Threshold "Duration % Of rigional (%)" and you must know that

Duration % of Origional (% )=  Actual Duration / Original Duration * 100

Now I Am not having a clear understanding that how P6 Calculates Actual Duration and Origional Duration Values.

Please Clear This Concept.

Thanks in Advance.

Arian