Forecast Calculation

Dear SAPper,

I am implementing forecast in my client.

My client keep asking about how the forecast calculating the figure.

I had looked for the calculation base on SAP help, but it seem i don't get the clue how SAP calculate it.

http://help.sap.com/saphelp_46c/helpdata/en/a5/6320e843a211d189410000e829fbbd/content.htm

Can you give me some clue about how SAP calculate forecast.

best regards,

Freddy Ha