Hi Pankaj,
To determine the expected consumption (extrapolations or proration), SAP uses this weighting key and after you maintain all the relevant parameters in spro, SAP internally applies this weighting procedure and determines the expected consumption, if you want to calculate yourself then you have to go through the SAP help links which I sent you earlier.
Weighting key has to be maintained for Billing Operands with Operand Category TQUANT, QUANT and AMOUNT - this is used in Billing if there is a need a need for proration (examples like change in Price, change in Tariffs during the billing period).
You have mentioned that you need weighting key at Register Data - I am not sure what do you mean by this? is this Rate Data you are referring to? if so then only Rate Type and Rate Fact Groups are maintained.
Thanks,
Srini