Calculation key in PS Network Activity

Hi

Can someone give an example of using the Calculation key in Network activity.

I am involved in PM module but time and again had to be involved with the PS team too.

I have read the help of the fields and its dependencies but still cannot understand its practical use.

I will appreciate if someone with values can explain  for instance using key "1 Calculate Duration", in which fields i need to enter the values and which field will provide the output.

Thanks

Faisal